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Advanced bainitic steels with the multiphase structure of bainitic ferrite, retained austenite and martensite exhibit distinctive fatigue crack initiation behavior during high cycle fatigue/very high cycle fatigue (HCF/VHCF) regimes. The subsurface microstructural fatigue crack initiation, referred to as “non-inclusion induced crack initiation, NIICI”, is a leading mode of failure of bainitic steels within the HCF/VHCF regimes. In this regard, there is currently a missing gap in the knowledge with respect to the cyclic response of multiphase structure during VHCF failure and the underlying mechanisms of fatigue crack initiation during VHCF. To address this aspect, we have developed a novel approach that explicitly identifies the knowledge gap through an examination of subsurface crack initiation and interaction with the local microstructure. This was accomplished by uniquely combining electron microscopy, three-dimensional confocal microscopy, focused ion beam, and transmission Kikuchi diffraction. Interestingly, the study indicated that there are multiple micro-mechanisms responsible for the NIICI failure of bainitic steels, including two scenarios of transgranular-crack-assisted NIICI and two scenarios of intergranular-crack-assisted NIICI, which resulted in the different distribution of fine grains in the crack initiation area. The fine grains were formed through fragmentation of bainitic ferrite lath caused by localized plastic deformation or via local continuous dynamic recrystallization because of repeated interaction between slip bands and prior austenite grain boundaries. The formation of fine grains assisted the advancement of small cracks. Another important aspect discussed is the role of retained austenite (RA) during cyclic loading, on crack initiation and propagation in terms of the morphology, distribution and stability of RA, which determined the development of localized cyclic plastic deformation in multiphase structure.

The development of ultrastrong maraging stainless steels (MSSs) is always in high demand. However, traditional high-strength MSSs generally exhibit early plastic instability with a low uniform strain since the precipitated nanoparticles are non-coherent with the body-centered-cubic (BCC) lath martensitic matrix. Here, we design a novel ultrahigh strength MSS (Fe-5.30Cr-13.47Ni-3.10Al-1.22Mo-0.50W-0.23Nb-0.03C-0.005B, wt.%) using a cluster formula approach. A fabulous microstructure consisting of a uniform distribution of high-density coherent B2-NiAl nanoprecipitates (3−5 nm) in BCC martensitic matrix was successfully obtained. This alloy has not only an exceedingly high ultimate tensile strength of 2.0 GPa, but also a decent uniform elongation of 4.2%−5.1%, which is almost triple of the value observed in existing MSSs. We present an in-depth discussion on the origins of ultrahigh strength and uniform plastic strain in the new alloy to validate our design strategy and further offer a new pathway to exploit high-performance MSSs.

A series of TA15 specimens with different process parameters were built by electron beam melting in this paper to systematically investigate the relationship of material-process-microstructure-mechanical properties. A quantitative mathematical model of yield strength and Vickers hardness with respect to α lath thickness was established. Microstructure analysis results showed that the microstructure of TA15 titanium alloy were basketweave microstructure, lamellar microstructure and bimodal microstructure depending on process. In general, the yield strength, Vickers hardness and α plate thickness are correlated with the Hall-Petch equation, but the mathematical model established in this paper has a better fitting effect. Process parameters affect the mechanical properties of the TA15 titanium alloy by affecting α lath thickness, and speed factor has the greatest influence on α lath thickness. The tensile properties of TA15 titanium alloy are as well as, if not better than, that of TA15 titanium alloys with tri-modal microstructure.

This study aims to compare the microstructure of 17–4 PH stainless steel (SS) manufactured via laser powder bed fusion (L-PBF) and laser powder directed energy deposition (LP-DED) in non-heat treated (NHT) and heat treated conditions. In addition, the room-temperature tensile behavior of heat-treated L-PBF and LP-DED 17–4 PH SS has been investigated and compared with that of the wrought counterpart with the same heat treatment conditions. The results show that the L-PBF specimens have a finer microstructure (ferrite + lath martensite) than the LP-DED ones (massive ferrite + Widmanstatten ferrite) in NHT condition. Electron backscatter diffraction analysis shows that the L-PBF and LP-DED specimens have twin-based substructure lath martensite after heat treatment. Despite the lower tensile strength of the LP-DED specimens compared with the L-PBF ones, the ductility of peak-aged LP-DED specimens was reduced due to the presence of the δ-ferrite phase having a significant plastic deformation incompatibility with the martensite.

Quenching and partitioning (Q&P) treatment is a novel method to produce advanced high strength steel with excellent mechanical properties. In this study, combination of multiple-cyclic annealing and Q&P process was compared with traditional cold-rolled Q&P steel to investigate the microstructural characteristics and austenite retention. The results showed that retained austenite in traditional Q&P sample was principally located in the exterior of austenite transformation products, while those in multiple-cyclic annealing samples were mainly distributed inside the transformation products. With the increase in cyclic annealing number, both of austenite fraction and austenite carbon content increased, attributing to higher initial austenite carbon content and larger number of austenite/neighbored phase interface to act as carbon partitioning channel. In traditional Q&P sample, the deformed ferrite was recrystallized by sub-grain coalescence, while the austenite was newly nucleated and grew up during annealing process. As a comparison, the ferrite in multiple-cycle annealing samples was formed by means of three routes: tempered martensite that completely recovered with retention of interior martensite variant, epitaxial ferrite that formed on basis of tempered martensite, ferrite that newly nucleated and grew up during the final annealing process. Both of lath martensite and twin martensite were formed as initial martensite and then tempered during partitioning process to precipitate e carbide with C enrichment, Mn enrichment and homogeneous Si distribution. Compared with the traditional cold-rolled Q&P steel, the Q&P specimens after multiple-cyclic annealing show smaller strength and much larger elongation, ascribing to the coarser microstructure and more efficient transformation induced plasticity (TRIP) effect deriving from retained austenite with high carbon content and larger volume fraction. The application of double annealing treatment can optimize the mechanical properties of Q&P steel to show a striking product of strength and elongation as about 29 GPa%, which efficiently exploit the potential of mechanical performance in low carbon steel.

The microstructure evolution and microhardness of Q&P980 steel heat-affected zone (HAZ) was investigated systematically using weld thermal simulation technique. The phase transformation temperatures were determined by the dilatometry. Ac1 and Ac3 were observed to be quite constant, 750 and 935 °C, at the heating rates higher than 150 °C/s. The different regions of HAZ were obtained varying the peak heating temperature in the simulation experiments. With the peak temperature increased from 300 to 1350 °C, the microstructure evolved from martensite/ferrite/retained austenite to tempered martensite/carbides/ferrite, then finally turned to single martensite phase with fine lath or coarse lath morphology. The volume percentage of retained austenite reduced dramatically from 13% to 2% due to retained austenite transformation at high temperature. The lowest microhardness of 268 HV was obtained in sub-critical HAZ and the highest microhardness of 485 HV was obtained in fine grained HAZ. The welding continuous cooling transformation (CCT) diagram of Q&P980 steel corresponding coarse grained HAZ was constructed by dilatometric methods. There was ferrite, bainite and martensite transformation regions when the cooling rates ranged from 0.1 to 100 °C/s. The microhardness kept at a high certain level of 450-460 HV at high cooling rates (≥20 °C/s) because of the formation of martensite. By the CCT diagram determined, the effect of cooling rate on microstructure and microhardness can be deduced and utilized for optimizing the welding parameters.

This study investigated the hydrogen-related fatigue fracture in 2Mn-0.1C steel having a lath martensite microstructure. The presence of hydrogen significantly reduced the fatigue life. The transgranular surface was a main component in each stress range of the uncharged specimen, while the intergranular surface was frequently observed in the hydrogen-charged specimen. The crystallographic orientation analysis by electron backscattering diffraction revealed that the cracks mainly propagated along {011} planes regardless of the presence of hydrogen. Compared with the uncharged specimen, however, plastic deformation was localized near the fatigue crack in the hydrogen-charged specimen. According to the reconstructed fracture process by fracture surface topography analysis, the hydrogen-related fatigue fracture was discontinuous and composed of isolated nucleation of intergranular cracks and quasi-cleavage crack propagation initiated at the pre-existing intergranular cracks.

The growth of austenite from as-quenched martensite during intercritical annealing of a medium-Mn steel was studied by numerical simulation and experimental verification. The thickening of the austenite lath was numerically simulated under the assumption of local equilibrium. According to the presence or absence of cementite and the nucleation position of austenite, three models are proposed: austenite forming on the martensitic lath boundaries (Setup 1), on martensitic lath boundaries with far-away cementite location (Setup 2), and on the cementite/martensite interface (Setup 3). Result shows that the growth kinetics of austenite under the three models are slightly different, mainly because cementite is thermodynamically unstable in the two-phase zone. In the Setup 2, the growth of cementite and austenite occurs simultaneously at the early stage, in which the austenite grows slowly compared to Setup 1, and when cementite begins to dissolve, the austenite grows faster. In the Setup 3, cementite grows first and then dissolves to provide solute carbon for austenite growth, leading to the fastest austenite growth. In addition, with larger initial size of martensite, both growth kinetics of austenite and cementite and the dissolution kinetics of cementite are slowed down. Combining with microstructure characterization with EPMA, TEM and dilatometric measurement, the Setup 2 is considered to be the optimal model in which austenite mainly grows at the martensitic lath boundaries rather than at the ferrite/cementite interface inside the laths. Moreover, in order to elucidate the influence of cementite dissolution on the growth of austenite, a round tube austenite forming on the cementite rod (Setup 4) was proposed. The growth of austenite is accompanied with the continuous carbon supply from the dissolution of cementite, which could increase the stability of reverse transformed austenite but has little effect on the increase of austenite volume fraction.

PurposeThe purpose of this paper is to investigate the effect of the soft annealing time on the microstructure and hydrogen embrittlement (HE) of Fe-0.22C-11.54Mn-2.05Al steels.Design/methodology/approachSteels A and B with different morphologies were prepared by cold rolling after warm rolling, long/short softening annealing and finally annealing at 700 °C for 30 min. Uncharged and charged samples were subjected to tensile, and HE behavior was studied by electron backscattered diffraction, scanning electron microscopy and X-ray diffraction.FindingsThe two samples exhibited similar tensile strengths. The homogeneous equiaxed microstructure of steel B was found to be more conducive to relieve its HE sensitivity. Steel A exhibited bimodal-grained microstructures – blocky and lath. The formation of crack in the blocky grains of steel A resulted in a significant reduction in its plasticity and tensile strength.Originality/valueThe high HE susceptibility of steel A is mainly connected with the inhomogeneity of martensite transformation.

In this paper, 2Cr13 stainless steels with different Mo contents (x = 0 wt%, 0.5 wt%, 1.0 wt%, 1.5 wt%, 2.0 wt%) were fabricated by Direct Laser Deposition (DLD) technology. The effects of Mo element on the microstructure evolution and comprehensive properties such as corrosion resistance and wear resistance of 2Cr13 stainless steel were investigated in detail. The results showed that the microstructure for the DLD 2Cr13 stainless steel was mainly composed of martensite, ferrite, and a small amount of Mo2C and Cr23C6 carbides. With the content of Mo increased from 0.5 to 1.5 wt%, the martensitic microstructure was refined and more dispersed Mo2C and Cr23C6 carbides were formed. The average microhardness and corrosion resistance of the DLD 2Cr13-xMo were increased with the Mo content increased from 0 to 1.0 wt%, and then decreased when the Mo content exceeded 1.0 wt%. For the DLD 2Cr13-1.0 wt% Mo, the average microhardness reached up to 620 HV0.2, and the highest corrosion resistance (self-corrosion potential − 0.34 V) was obtained, in which the self-corrosion potential was 0.05 higher than that of DLD 2Cr13 stainless steel. The wear resistance of the DLD 2Cr13-xMo gradually increased with the increase of the Mo, the wear mass loss of DLD 2Cr13-xMo (x = 1.0 wt%) was only 37% that of DLD 2Cr13 stainless steel. Schematic diagram of DLD process and the microstructure, properties of 2Cr13 stainless steel with different Mo contents fabricated by DLD technology: (a)The DLD process diagram, (b)The changes of samples microstructure with different Mo contents. The fabrication process of 2Cr13 stainless steel with different Mo content by Direct Laser Deposition and the microstructure evolution process of samples with different Mo content are visually described in this graph. Fig. 1a illustrates in detail the DLD process design for sample preparation using FL-Dlight02-3000W semiconductor laser (maximum power 3000 W, spot size 4mm×4 mm). Fig. 1b shows that the microstructure in the DLD 2Cr13 stainless steel was mainly composed of martensite, ferrite, and a small amount carbides. Combined with the principle of DLD and the effect of Mo addition on microstructure, it is concluded that when Mo content is 1.0wt%, the martensite grain size is smaller, the lath is parallel and the growth direction is consistent, the content of ferrite and carbide is not much, which leads to the highest microhardness and the best corrosion resistance and wear resistance.

In this study, the short-term aging was carried out to reveal the evolution of precipitates and mechanical properties of heat resistant 9CrMoCoB steel during the early creep, replacing the conventional creeping. The tempered martensite lath structure (TMLS) and precipitates were observed in the as-aged 9CrMoCoB steel. TMLS in the matrix underwent a transition to the polygonal ferrite after aging only for 300 h. In comparison, the mean diameter of the precipitates increased from 183 to 267 nm after aging at 650 °C for 300 h. Also, the mean diameter of the precipitates increased from 183 to 302 nm at 700 °C. The room-temperature and high-temperature strength of 9CrMoCoB steel decreased after high-temperature aging, which may be mainly due to precipitates coarsening. Many M23C6 phases precipitate in the prior austenite grain boundary (PAGB) and lath boundary. After aging 100 h, TMLS transformed into polygonal ferrite, and the size of the precipitate at the subgrain boundary was about 100 nm, while after 300 h of high-temperature aging, large precipitates appear (400 nm) in the matrix. After 200 h of high-temperature aging, the obvious growth of precipitates on the PAGB and lath boundary weakens the pinning effect on the PAGB and martensite lath boundary and accelerates the transformation of microstructure and mechanical properties.

Through the deep medium-temperature tempering treatment, microstructure observation and hardness test of the carburized M50NiL bearing steel, the influence of the temperature and times of deep tempering on the microstructure and hardness of the steel were studied. The results show that the deep medium-temperature tempering is beneficial to the dissolution of large-size carbides in the carburized layer of the steel at the initial state, uniform distribution of carbides, and the precipitation of granular carbides from lath martensite. During deep tempering treatment, some granular carbides are connected to form rod-like carbides. With the increase of the number of deep tempering, the lath martensite in the steel is decomposed and the size is obviously refined, and more carbides in the core area are precipitated, whose distribution is more dispersed uniform. The deep tempering at 460 °C and 480 °C can slightly increase the surface hardness of bearing steel, but five times of medium temperature tempering at 500 °C makes the hardness of the steel slightly decreased. There exists the lowest point of hardness in the subsurface layer of the initial-stated M50NiL steel, which is however eliminated after 5-time deep tempering at 460 °C and 500 °C, and this is related to the elimination of stripe-like carbides after the two deep tempering processes.

Drop-tube processing was used to solidify rapidly a congruently melting, single phase intermeallic Ni5Ge3. This process results in the production of powders with diameters that are between 850–53 μm. After etching that occurs at low cooling rates (850–150 μm diameter particles, 700–7800 K s–1), an isolated plate and lath microstructure in what is an otherwise featureless matrix constitutes the dominant solidification morphology. By contrast, when the cooling rates are higher (150–53 μm diameter particles, 7800–42 000 K s–1), it is isolated hexagonal crystallites within a featureless matrix, which constitute the dominant solidification morphology. The TEM analysis of selected area diffraction shows that plate and lath microstructures are variants of e-Ni5Ge3, which are partially ordered. By contrast, the isolated hexagonal crystallites are revealed to be disordered. However, the featureless matrix of both microstructures are the fully ordered variant of the same compound. The plate and lath has a very different EBSD and GOS signatures to the hexagonal crystallites structure. Histogram of the correlated grain orientation angle distribution across grain orientation in plate and lath microstructure sample from the 300–212 μm fraction showing predominance of low angle grain boundaries. However, grain orientation in isolated hexagonal crystallites from 150–106 μm revealing the distribution typical of random grain orientation.

The local plastic strain evolution associated with crack growth in hydrogen-assisted quasi-cleavage fracture was investigated using tempered lath martensitic steels. The quasi-cleavage crack grew via the following process. After crack initiation, the crack grew sharply to a certain length by repeated episodes of nano-void nucleation and coalescence. When the sharp crack tip intersected microstructural boundaries such as a lath or block, crack deflection/branching occurred. This was followed by crack tip blunting, which temporarily stopped crack growth. Further crack growth was possible via one of the following two routes: (1) sharp crack initiation/growth from the blunt crack tip, and (2) new crack initiation near the blunt crack tip. The newly formed cracks subsequently coalesced. Repetition of this multi-scale crack growth mechanism finally caused a quasi-cleavage fracture. Correspondingly, hierarchical crack morphologies were observed, which coincided with the lath martensitic microstructures and fractographic features. Furthermore, specific correlations were found between hydrogen-assisted cracking behavior and local plastic strain evolution at different spatial scales. Specifically, the largest plastic strain evolution occurred in the region where crack coalescence was observed. The second largest plastic strain evolution occurred when crack tip blunting occurred. Nanoscale local plasticity evolution around a sharp crack was also observed as an appearance of intense slip bands, indicating that the local plasticity played a key role in the hydrogen-related sharp crack growth.

$$\alpha + \beta$$ titanium alloy is known for its high strength to low weight ratio, excellent mechanical properties and superior corrosion resistance, which make it a favorable material for aerospace and biomedical applications. However, on exposure to oxygen and nitrogen at elevated temperatures, a hard and brittle $$\alpha$$ -layer develops on the titanium alloy surface, which is referred to as ‘ $$\alpha$$ -casing.’ In the present study, Ti–6Al–4 V was heat treated above the $$\beta$$ -transition temperature and cooled at different rates in water, air and furnace to obtain lamellar morphologies of different $$\alpha$$ -lath thicknesses. Tensile and compact tension (CT) specimens with three different lamellar morphologies were obtained by heat treatment in such a way that one set (A) had ‘ $$\alpha$$ -case’ and other set (B) was devoid of such casing. Water quenched (WQ) samples formed a very thick $$\alpha$$ -case with numerous micro-cracks compared to the air and furnace cooled samples. On tensile loading, the WQ sample with $$\alpha$$ -case failed in fully brittle mode and the ductility of the air-cooled (AC) and furnace cooled (FC) samples with $$\alpha$$ -case was reduced significantly. It was also observed that the fatigue crack growth rate (FCGR) increased considerably as the apparent effective crack front area was reduced due to $$\alpha$$ -case formation.

This study involved a commercial hot-rolled dual-phase (DP) steel consisting of martensite (~10 pct) and ferrite phases. The harder lath martensite was located at the grain boundaries and triple junctions of the equiaxed ferrite grains. Tempering and high-pressure torsion (HPT) were used to alter the phase hardness differential ∆H (where $$ \Delta H = H_{\text{Martensite}} - H_{\text{Ferrite}} $$ ) of the DP. The relationship between ∆H and non-uniform elongation, eNU, or post-necking ductility under tensile deformation, was then explored. Tempering softened predominantly the martensite, while HPT increased the ferrite hardness. Both led to a reduction in ∆H. A drop in ∆H in the tempered DP resulted in a steady increase and eventual saturation in eNU. On the other hand, a ∆H decrease in the HPT specimens showed an initial increase in eNU followed by a drop. Strain analysis, with optical digital image correlation during tensile deformation of the tempered DP samples, clearly related the formation of strain localization with ∆H. In particular, severity of strain localization during necking scaled linearly with ∆H. This study thus brought out a potential relationship among the phase hardness differential (∆H), severity of strain localizations and post-necking ductility (eNU).

The effect of Cu-riched clusters on strength and impact toughness in a tempered Cu-bearing high-strength low-alloy (HSLA) steel is investigated. With increasing the tempering temperature, it is found that the yield strength increases firstly, achieving the maximum value (~ 1053 MPa) at the tempering temperature of 450 °C, and then decreases significantly with the rise of tempering temperature. The tempering temperature-dependent yield strength is closely related to the precipitation of Cu-riched clusters. When tempering at 450 °C, the peak strength will be reached as the nanoscale Cu-riched clusters with small size and high number density will cause a strong precipitation strengthening (~ 492 MPa) due to the dislocation shearing mechanism. However, the Cu-riched clusters will coarsen with further increasing tempering temperature, resulting in obvious decrement of yield strength owing to the dislocation bypassing mechanism. Compared with the yield strength, the variation in impact energy displays an inverse tendency and the impact energy is only 7 J for the sample tempered at 450 °C. The fracture mode can be well explained by the competition between the cleavage fracture strength (σF) and “yield strength” (σY). Although transgranular cleavage fracture can be found in samples tempered at 450 and 550 °C, the crack propagation along the lath boundaries is prevented in the sample tempered at 550 °C. The reason is that the number density of Cu-riched clusters at lath boundaries decreases and the segregation of Mo element at the lath boundaries is induced, which will increase the bonding energy.

With the decreasing of finish rolling temperature, the microstructure of the hot rolled steel was changed from lath martensite to complex microstructures composed of lath martensite, ferrite, granular bainite and lath bainite, unrecrystallized prior austenite grains are the main reason for the formation of complex microstructures. After tempering, the steels are mainly composed of ferrite and cementite. For the steel with high finish rolling temperature, the proportion of high misorientation boundary is less, and it has rod-like precipitates with the length of 25−85 nm and punctate precipitates with the size of 4−16 nm, the intensities of {111} and {001} textures are enhanced. The experimental steel has the best combination with strength and toughness as yield strength of 996 MPa, tensile strength of 1023 MPa, total elongation of 19.1 %, impact value at -60 ℃ of 125 J. Grain refinement, higher density of dislocation, fine and dispersed precipitates could enhance the yield strength and tensile strength. Stronger intensity of {332} texture and weaker intensity of {001} texture could improve the impact toughness.

As a kind of high strength microalloyed steel, pipeline steel has high strength and toughness. However, the formation of the intercritically reheated coarse-grained heat affected zone (ICCGHAZ) during the welding process is an important limitation affecting its performance. Herein, microstructural transformation in the ICCGHAZ of X100 pipeline steel after a secondary thermal cycle and hydrogen sulfide stress corrosion cracking (SSCC) resistance of the transformed microstructure are investigated. The microstructure of X100 pipeline steel after the secondary thermal cycle is similar to that after the primary thermal cycle, comprising lath bainite and granular bainite. With an increase in t8/5 (time required for the material to cool from 800 °C to 500 °C), the M-A constituents that continuously precipitate along the prior austenite grain boundaries in the second thermal cycle are coarsened and form a necklace-type structure. A variation in t8/5 does not significantly affect the crystallographic characteristics of the ICCGHAZ. The stress corrosion test shows that the resistance to SSCC decreases and cleavage fracture characteristics become more noticeable with an increase in t8/5.

An approach to the assembly of wood laths using the press with a new design is considered. The suggested approach decreases the duration of piezo treatment due to separation of the assembly process into two stages, namely, blocking of glued surfaces in a press and with termination of product gluing outside of a press until full polymerization of glue is achieved. A design of a press for implementation of the suggested assembly approach is suggested, and analytical relationships and results of experimental studies are given, which confirm the possibility of lath assembly on the equipment with a reduced travel period.

Mechanical characteristics of an alloy, used for manufacturing parts of heavy-duty machines, operating under conditions of dynamic and cyclic loads, should meet increased requirements. According to the concept of lean alloying, quality of an alloy and highest possible properties of a chemical composition are determined by the modes of heat treatment. Results of a study of quenching temperature effect on microstructure and properties of a new lean alloyed steel MAGSTRONG 450L having the following chemical composition, %, mass: 0.21 С; 0.23 Si; 0.96 Mn; 0.03 Cr; 0.41 Ni; 0.023 Ti; 0.0021 B presented. Smelting of ingots of a given chemical composition was carried out in a vacuum induction furnace ZG-0.06L. Reduction of ingots was carried out on a hydraulic press П6334 and on a single-stand reversing hot rolling mill 500 duo. The samples were heated in a ПКМ 3.6.2/12,5 chamber furnace. Best mode of the hardening heat treatment of sheet rolled product of steel MAGSTRONG H450L was established, ensuring forming of a mixed structure, comprising 80% of lath martensite and about 20% of high temperature lamellar martensite. Such a structure ensures maximum values of impact toughness KCV–40 = 38 J/сm2 at high strength parameters (tensile strength σв = 1514 МPa, offset yield strength σ0,2 = 1243 МPа, hardness 460 HBW) and satisfactory indices of plasticity (percent elongation δ50 = 16 %) which correspondents to requirements ТС 14-101-1034–2015. Based on results obtained, recommendations were elaborated for heat treatment of the sheet steel MAGSTRONG H450L, thickness 8–20 mm as per OJSC “BELAZ” order. Application of the obtained results of the study will enable to create and perfect the technological modes of heat treatment of alloys with similar composition.

The segregation behavior of trace amount of phosphorus (P) and the mechanical properties of dual-phase (DP) steels have been systematically studied. The microstructure of DP steels is mainly composed of martensite, ferrite and nanoscale carbides. For the DP steels with different trace amounts of P (≤ 0.015 wt%), P has almost no effect on the mechanical properties. Atom probe technology (APT) analyses confirm that P segregation was only found at the precipitate/matrix interface. Moreover, the precipitates of (Ti, Mo) C are widely distributed in the ferrite, martensite and ferrite/martensite interface regions. The special segregation feature of P would not concentrate at specific regions such as ferrite/martensite interface and/or martensite lath interface, which reveals that trace amounts of P (≤ 0.015wt%) have almost no effect on the mechanical properties of DP steels. It is proved for the first time that the MC-type carbides of (Ti, Mo) C can reduce or eliminate the damage effect of P on the mechanical properties of steels, which provides a new way for the design of alloys to reduce P damage. This work will promote to increase the P content control standard in DP steels from 0.01 to 0.015 wt%, which will not change the mechanical properties, but greatly reduce the scrap rate and increase the energy efficiency of the manufacturing process.

In the present work, Tungsten (W)/oxide dispersion strengthened (ODS) steel joints were fabricated by the direct solid state diffusion bonding (SSDB) technology with a multistage cooling process, and the microstructure and tensile strength of the bonded interfaces and parent materials were experimentally investigated. The results show that W and ODS steel can be successfully bonded at the temperature ranging from 900 °C to 1050 °C, without severe macroscopic deformation or obvious microscopic defects. Reaction layers generated at the bonded interfaces are evolutive with the bonding temperature, result in different fracture locations of the bonded joints. In the joint bonded at 950 °C, a higher interfacial strength of ~ 234.2 MPa is achieved, due to the formation of nano-scale intermetallic compound FeW. Microstructure of W remains stable after all the SSDB processes, while the lath structure of ODS steel is completely broken and transformed into the equiaxed grains, which should be responsible for the deterioration of strength. When the bonding temperature is higher than 950 °C, the pinning effect of precipitates M23C6 and nano-oxide particles on the movement of dislocations is observed.

0.3%C-CrMoV, a medium-carbon high-strength low-alloy steel, is proposed to be used in the fabrication of rocket motor cases for satellite launch vehicles. Gas tungsten arc welding (GTAW) is used in the fabrication of motor cases. In the present study, GTAW with different pre- and post-weld heat treatments (PWHT) was carried out in annealed, hardened and tempered heat-treated conditions. PWHT employed was hardening at 920 °C-1 hr-oil quenching (OQ) and tempering at 505 °C-2 hr-OQ. Tensile strength and fracture toughness of the weldments were evaluated in each of these conditions. The effect of copper coated filler wire on the mechanical properties of the weldments has also been investigated. Welding in the annealed condition followed by PWHT resulted in nearly 100% weld efficiency on tensile strength and nearly 90% on fracture toughness with respect to the quench and tempered mechanical properties of the parent metal. The microstructure of welds is predominantly lath martensite with characteristic multivariant carbides in PWHT condition. Lower fracture toughness in as-welded condition is attributed to high dislocation density, presence of twins and presence of untempered martensite. Highest fracture toughness and tensile strength in PWHT conditions are attributed to heterogenous microstructure (mixture of lath martensite and bainite,10-15% volume fraction), partial recovery of lath structure, and reduction in dislocation density and presence of fine alloy carbides. The fractography of tensile tested specimens revealed a mixed-mode failure characterized by dimples, ductile tear ridges and few cleavage facets in the weld coupons subjected to PWHT.

Medical-grade Ti6Al4V extra-low interstitial (ELI) alloy has widespread applications in the biomedical industry due to its excellent corrosion and wear resistance. Even though 3D printing offers geometry flexibility and rapid means of fabricating customized parts, 3D-printed parts are often plagued with defects including porosity, high residual stresses, and non-equilibrium structures. Thus, post-processing heat treatments may be required to optimize its properties for engineering applications. In this study, the effect of post-processing heat treatment on the microstructure, hardness, and wear properties of 3D-printed and conventionally produced medical-grade Ti6Al4V ELI alloy samples was investigated. In general, distinct α (alpha) lath and basket-weave lath structures with a high degree of orientation were observed within the microstructure of the as-printed samples. Heat treatment led to the growth of distinct continuous and discontinuous α-lath structures along prior β (beta) grain boundaries as well as basket-weave lath and the coalescence of V-shaped structures within the prior β-grains. The hardness of both the 3D-printed and conventionally produced samples increased after heat treatment (≥ 400 HV), regardless of the cooling rate and aging temperature. After being water quenched/aged, the 3D-printed samples at 500 °C had the highest hardness values owing to the presence of coarse V-shaped structures. Furthermore, the V-shaped structures were always harder than all other structures regardless of the heat treatment and manufacturing process used, indicating that these structures dictate the overall mechanical integrity of the material. X-ray diffraction and electron probe microanalysis indicated that the V-shaped structures are rich in aluminum and titanium content, which can form hcp α′ (AlTi3) intermetallic phases. The 3D-printed samples had higher wear resistance overall than the conventionally produced samples regardless of the heat treatment used. Aging at 500 °C led to a higher coefficient of friction after 3D printing owing to an increase in α-phases. Therefore, during heat treatment, the microstructure and properties of medical-grade Ti6Al4V ELI alloy are significantly affected by the starting microstructure, the rate of cooling below the β-transus, and aging temperature and time, regardless of the manufacturing process used.

Intense pulsed ion beams (IPIB) have been applied on the surface of a lath martensitic steel with aim to improve its hydrogen embrittlement resistance and reveal the key cmaterial factors leading to failure. Hydrogen charging slow strain rate tensile tests show that IPIB can increase the ultimate fracture strength. The main fracture mode changes from intergranular fracture (untreated) to quasi-cleavage fracture (treated). Atomic probe tomography reveals that C atoms segregate at prior austenite grain boundaries for the untreated steel. After IPIB treatment, the C content at the PAGBs is reduced and high-carbon martensite forms in the treated layer, which improves the HE resistance. This study suggests that C segregation at grain boundaries is one of the main factors to cause the high HE susceptibility for the investigated lath martensitic steel and C segregation should be avoided when developing high strength steels with high HE resistance.

The martensitic hot-rolled 0.3C-6Mn-1.5Si (wt%) steel was annealed at 630 °C for 24 h to improve its cold rollability, followed by cold rolling and annealing at 670 °C for 10 min. The annealing process was designed based on the capacities of industrial batch annealing and continuous annealing lines. A duplex submicron austenite and ferrite microstructure and excellent tensile properties were obtained finally, proved the above process is feasible. “Austenite memory” was found in the hot-rolled and annealed sample which restricted recrystallization of lath martensite, leading to lath-shaped morphology of austenite and ferrite grains. “Austenite memory” disappeared in the cold-rolled and annealed sample due to austenite random nucleation and ferrite recrystallization, resulting in globular microstructure and refinement of both austenite and ferrite grains. The austenite to martensite transformation contributed most of strain hardening during deformation and improved the uniform elongation, but the dislocation strengthening played a decisive role on the yielding behavior. The tensile curves change from continuous to discontinuous yielding as the increase of cold-rolled reduction due to the weakening dislocation strengthening of austenite and ferrite grains related to the morphology change and grain refinement. A method by controlling the cold-rolled reduction is proposed to avoid the Luders strain.

The formation mechanism of the reversed austenite and the interaction between austenite inversion and plastic deformation in the G520 martensitic stainless steel welds was studied by adjusting the tempering holding time during post-weld heat treatment. The results show that the weld microstructure has consisted mainly of tempered lath martensite, reversed austenite, and grain boundary precipitation carbides. The volume fraction of the reversed austenite has a great influence on the mechanical properties of the weld. The nucleation position and size of reversed austenite formed at the tempered martensite laths sub-block boundaries or austenite grain boundary are strictly restricted by the orientation of crystal attributing to the parallel orientation relationship between {110} M and {111}A. The reversed austenite with low dislocation density is mostly flakiness particle morphology, which greatly refines the tempered martensite laths. The transformation of martensite into reversed austenite is a diffused phase transformation, and the increase in reversed austenite is consistent with the increase in Mn content and Mo content reduction in the weld. For weld with Mn 0.87 wt.% and Mo 1.16 wt.%, the volume fraction of reversed austenite is 14.53%, and the welds exhibit excellent mechanical performance.

Based on the thermal simulation method, a systematical analysis was conducted on the effect of welding peak temperature and the cooling time that takes place from 800 to 500 °C on microstructure, precipitates, substructure and microhardness of the reheated coarse-grained heat-affected zone (CGHAZ) of G115 novel martensitic heat-resistant steel. As revealed from the results, the microstructure of un-altered CGHAZ (UACGHAZ) and supercritically CGHAZ (SCCGHAZ) was lath martensite, and structural heredity occurred. Intercritically reheated CGHAZ (IRCGHAZ) exhibited martensite and over-tempered martensite, and subcritical CGHAZ (SCGHAZ) displayed martensite and under-tempered martensite. The austenite in UACGHAZ and SCCGHAZ was transformed with the diffusion mechanism during the first thermal cycle, but with the non-diffusion mechanism during the second thermal cycle. For this reason, Ac1 and Ac3 during the second thermal cycle were significantly lower than those during the first thermal cycle, and Ac1 and Ac3 were reduced by nearly 14 and 44 °C, respectively. Since the content and stability of the austenite alloy during the second thermal cycle of IRCGHAZ were lower than those during the first thermal cycle, Ms increased by nearly 30 °C. There were considerable precipitates in the over-tempered region of IRCGHAZ, and the Laves phase was contained, which was not conducive to high-temperature creep property. Moreover, the dislocation density and the number of sub-grains in the region were lower, resulting in a sharp decrease in the microhardness, and it was the weak area in the reheated CGHAZ.

This work investigated the evolution of multiphase microstructure and impact fracture behavior of medium carbon high silicon high strength steel subjected to the austempering treatment at 240, 360, and 400 ℃. The results show that martensite, bainite, and retained austenite (RA) are the main microstructural phases. The austempering treatments at 360 and 400 ℃ caused the formation of carbon-poor ferrite in the matrix, and the transformation of ultrafine bainite into coarse lath bainite and granular bainite, respectively. Thick filmy RA was distributed between bainite laths. The polygonal martensite-austenite islands and blocky RA formed along the grain boundaries. The average carbon concentration in the matrix decreased with the temperature increase, while the impact toughness initially increased and then dropped with temperature. The quasi-cleavage brittle fracture dominated the impact fracture mechanism of the sample austempered at 240 ℃ by forming tearing surfaces and tearing steps. The microcracks disappeared in the RA on the prior austenite grain boundaries. On the other side, the fracture surface of the sample austempered at 360 ℃ exhibited ductile fracture with deep dimples and brittle fracture with cleavage river patterns. The polygonal martensite-austenite islands or blocky RA constrained the microcracks. After austempered at 400 ℃, the brittle fracture was dominant, showing river patterns, and the microcracks propagated through the granular bainite without any resistance.

In this paper, the TRIP590 steel was used for C-Mn partitioning. The influence of C-Mn partitioning on the microstructure and mechanical properties of the steel was studied. SEM, EPMA, XRD and tensile tests were used to characterize the microstructure of the tested steel, calculate the content of retained austenite, and analyze the enrichment of C atoms and Mn atoms and mechanical properties. The results show that there was a lot of lath martensite and scattered ferrite in the microstructures of the Q&P steel and C-Mn partitioning steel. After C-Mn partitioning, the content of ferrite was increased. The enrichment of C and Mn in the C-Mn partitioning steel was relatively apparent, and the concentration of the atoms in the center of martensite was significantly higher than at the boundary between martensite and ferrite. Mn-rich areas were also C-rich areas. Compared with the Q&P steel, the C-Mn partitioning steel had a larger amount of retained austenite, higher elongation and PSE.

The presence of strain-induced phases formed in the course of ausforming step has been proven by thermodynamic calculation, microstructural observation and hardness examination, besides that, the effect of ausforming parameters on isothermally transformed bainite during austempering step was also essentially investigated. The results indicated that the types of strain-induced phases were determined by ausforming temperature and strain, for instance, the trace of strain-induced martensite could be detected in the sample subjected to ausforming at 300 °C for 20% strain. However, ausforming would introduce strain-induced bainite at a higher ausforming temperature (500 °C) instead of strain-induced martensite, which could gradually increase the average thickness of bainitic ferrite lath and impair bulk hardness with raising ausforming strain. Although ausforming process lessened the maximum attainable volume fraction of bainitic ferrite formed during austempering stage due to austenite mechanical stabilization, the existence of strain-induced martensite and the refined bainitic sheaves would make a contribution to hardness improvement when ausformed at 300 °C.

To study the effect of beam oscillating behavior on pore inhibition and microstructure evolution, laser oscillating welding of Q235 steel was carried out. The pore formation, microstructure evolution and mechanical properties were investigated. The pore formation was inhibited at frequency lower than 75 Hz and radius larger than 0.75 mm. The weld microstructure was mainly composed of blocky martensite (MB), lath martensite (ML), and ferrite. Under given radius of 0.75 mm, the fraction of the ML first increases and then decreases with the frequency increasing. Under given frequency of 75 Hz, the fusion zone is almost composed of MB while the ML disappears with the radius increasing to 1.25 mm. The weld microstructure was well accordance with the mechanical properties. The larger the fraction of the MB, the higher the micro-hardness and the tensile strength. The oscillating parameters were optimized as radius of 0.75 mm and frequency of 50 Hz. In this case, the weld is mainly composed of ML free of pores, with the elongation rate 15.5% higher than that without beam oscillation. Finally, the pore formation and inhibition mechanisms were illustrated by the Reynolds model basing on the melt stirring theory, and the microstructure evolution was discussed basing on the solidification theory.

In this paper, the microstructure evolution of TC17 titanium alloy prepared by laser stereo forming (LSF) was investigated. Forged TC17 plate was selected as the base material, and TC17 titanium alloy spherical powder with the size of 50-150 μm was the candidate cladding material. The microstructure in deposited TC17 titanium alloy was consisted of three parts, namely fine grains at the bottom of the deposition area, coarse β columnar crystals at the middle, while coarse β columnar crystals or equiaxed crystals at the top. The microstructure is significantly affected by the thermal cycle history during the LSF process. At different locations of the deposited samples, the composition, morphology, size and volume fraction of the phases in the grains are discrepant, as the metastable β phase and the acicular martensite (α′) structure were obtained at the bottom and in the middle, respectively. Due to thermal cycle during multi-layer deposition, the metastable β phase will gradually evolved into α + β phases. As the subsequent deposition process continued, the temperature in the middle of the deposition area decreased below the transformation point, and α′ is decomposed into α + β. The precipitated α phase gradually grew up under the action of thermal cycle. Meanwhile, the fine secondary α phase precipitated from the metastable β phase, and finally the coarser α lath and inter lath were obtained β structure.

Medium Mn steels, one of the most promising 3rd generation advanced high strength steels (AHSS), achieve an encouraging trade-off between the outstanding mechanical property and the production cost. As a typical medium Mn steel, 7Mn steels have superior mechanical property but their poor cross-tension property becomes the Achilles' heel, hindering the application in the automotive industry. The current study focuses on the cross-tension property of the resistance spot weld of 7Mn steel. Generally, martensite is produced in the nugget during the resistance spot welding (RSW). However, the microstructure in the weld nugget can be correspondingly tuned by directly optimizing the welding parameters. With post-weld pulses, in situ tempering occurs, which can decrease the segregation of Mn existing along martensite lath boundaries and facilitate the microstructure transition from martensite to tempered martensite. The tuning on the nugget microstructure facilitates the increase of cross-tension strength (CTS) from 1.5 to 3.7 kN. Although both cases fail in an interfacial fracture mode, a partial ductile fracture is demonstrated in the specimen with post-weld treatment, which is attributed to the occurrence of low-carbon α phase and second phase particles. This study elucidates that the decrease of segregation and the microstructure transition in the nugget are the dominant factor determining the CTS. It is therefore demonstrated that the reduction of Mn segregation and the formation of tempered martensite can increase the weldability of RSW joints of the medium Mn steels.

The fast development of various additive manufacturing processes is motivating research to improve microstructure control during laser assisted treatments, in order to spatially vary material properties on engineering components. In the present work, we demonstrate that a dual-pass laser treatment applied to a low carbon lath martensitic steel can simultaneously repair solidification or fatigue cracks, reduce surface roughness, and, most importantly, trigger phase transformations to create a more damage-resistant microstructure, namely, one that includes interlath austenite. We investigate these influences employing various scanning electron microscopy and transmission high energy X-ray diffraction analyses, mechanical characterization tests, and discuss the underlying mechanisms.

In the process of fatigue loading, the segregation of solid solution elements makes dislocations continuously pile-up in ferrite lath to form subgrain, which improves the fatigue properties. In this study, the low cycle fatigue properties of two kinds of Cr-Mo tempered martensitic steels with different grain sizes were investigated at 350 °C. Then, the fracture morphology and the subgrain of ferrite lath were analyzed. The results show that the low cycle fatigue life of 90H steel is significantly higher than that of 80H steel, and a large number of secondary cracks are found in the fatigue fracture. The reason is that the segregation of solid solution atoms affects the dislocations movement during low cycle fatigue. And the finer lath subgrain structure of 90H steel makes it easier to occur in grain. Thus, the evolution process of subgrain structures is changed, and the crack propagation is hindered, resulting in a large number of secondary cracks, which improves the fatigue life. In addition, the segregation effect on fatigue life decreased with the strain amplitude increasing.

The growth behavior and crystallographic orientation of bainite in a low carbon steel were investigated by a combination of in situ observation and electron backscattered diffraction. The intersection of different bainitic ferrite (BF) laths was analyzed in detail. The results show that the growth of a given lath stops when it runs into a preexisting lath of bainitic ferrite. Growth resumes, after a short stasis, through the nucleation of a new lath on the other side of the obstructing lath. The newly nucleated lath has been termed BF-clone because it shares the same crystallographic orientation with the prior bainitic lath. The nucleation of the BF-clone is promoted by the accumulated transformation strain in the region where the growing lath intersected the pre-existing lath. During the growth of the bainitic ferrite laths, the transformation strain was accommodated through the formation of twin-related variant pairs, which formed one after another with the identical lengthening rates.

Primary Al13Fe4 phases tend to grow along the [010] orientation forming coarse needles, flakes and lath shapes in hypereutectic Al–Fe alloys, which seriously deteriorates the mechanical properties of the alloys. Rare earth element modification is an effective method to refine the Al13Fe4 phase. Herein, the modification mechanism of the element Y on the primary Al13Fe4 phase was investigated using experimental and theoretical calculations. Addition of 0.5 wt% Y had the best modification effect on the primary Al13Fe4 phase. Y atoms mainly formed the ternary phase of Al–Fe–Y in the final solidification stage, and no Y element was found in the primary Al13Fe4 phase, eutectic Al13Fe4 phase and α-Al phase. Y entered the primary Al13Fe4 phase and preferably occupied Al sites, but increased the formation enthalpy of Al13Fe4. When Y atoms substituted the partial sites of Al atoms in the Al13Fe4 (010) surface, the surface energy reduced and the stability of the (010) surface increased. Therefore, surface doping of Y atoms could slow down the growth rate of the primary Al13Fe4 phase along the [010] orientation. Furthermore, the adsorption of Y on the (010) surface was an exothermic reaction, in which Y easily adsorbed on the (010) surface. The adsorption of Y changed the (010) surface structure and destroyed the advantage of selective growth along the [010] orientation. Comparatively, the adsorption mechanism was more likely to be the main reason for the morphological modification of the primary Al13Fe4 phase with the Y element. These calculations revealed the refinement mechanism from a thermodynamic viewpoint.

P92 steels (9 Cr – 0.5 Mo – 1.8 W) are one of the candidate materials for Ultra Super Critical (USC) power plants. Since normalization of P92 results in the formation of martensitic microstructure, the steel is generally used in the normalized and tempered condition. In the present work, the material is subjected to normalization at 1060 °C and the effect of tempering time has been studied at constant tempering temperature of 780 °C. Decrease in dislocation density, increase in lath width, and precipitate size are the predominant microstructural changes. Short term (∼500 h) stress rupture tests are conducted at 600 °C temperature and structure – property correlation is attempted.

The heat treatment and quenching process provides tremendous advantages for the steels by modifying the mechanical properties and affecting phase change in the structure of steel. So we perform this heat treatment process in various quenching medium to test the EN8 steels. Few specimen of EN8 steel were studied and analyzed after heating them in the Muffle Furnace between 750 °C and 950 °C followed by quenching in different medium. Oil, Water, and Air are used as different quenching medium. Using the Rockwell hardness equipment the mechanical properties such as hardness are measured, and the hardness of the quenched material is significantly greater than the parent material. The samples are then subjected to Charpy impact testing and Rockwell hardness testing after heat treatment. The sample was crated for Charpy test as per ASTM-E23, and for Rockwell hardness test as per ASTM-E18.In this work EN8 a grade of medium carbon steel is used. The composition of this EN8 steel is shown in the table below. The specimen was taken in the form of square bars 14 × 14 mm cross section which was then machined on lath machine by facing to the cross section of 10x10 mm and then cut. The final dimension of the specimen was 10x10x55 mm as per ASTM-E23. A U - notch of 2 mm depth was cut at the centre for impact testing.

Comparative analysis was conducted to illustrate the effect of high-temperature reduction pretreatment on the internal quality of 42CrMo casting billet. Analysis of macrostructure clarifies that reduction pretreatment could reduce the size of central cavities and equiaxed crystals and improve the segregation degree and porosity on statistics. Lath bainite matrix and granular structure on segregation bands mainly dominate the microstructure of original billet, while pro-eutectoid ferrite and its accompanying troostite are added due to the application of reduction pretreatment. Additionally, C, Mn, and Mo are suggested to be enriched in segregation bands where pores and inclusions tend to be generated. By clarifying the microstructure evolution during cooling process, physical compression and multiplication of prior austenite grain boundaries caused by recrystallization are regarded as the main reasons in improving the internal quality of 42CrMo billet.

This present work is the first investigation on the laser welding of the novel hot-stamped steel with 2000MPa (HSS2000). Softening was present in mixed-grain HAZ and tempered zone, due to martensite transformation, precipitated carbons and partial recovery of martensite lath. Tensile strength of welded joint was ∼1500 MPa, 80% of base metal, due to the presence of soften zone. However, the softening cannot be eliminated with decreasing laser heat input but it did not influence the tensile strength of welded joints. Thus, the laser welding of HSS2000 is acceptable in the heat input range of 67∼86 J/mm because the tensile strength can reach 1500 MPa, which is superior to other ultra-high strength steels at present. HSS2000 has a potential application in the automotive manufactures in the future.

Corrosion and stress corrosion cracking (SCC) behavior of some low-alloy high-strength steels microalloyed with Nb and Sb were in detail studied in a simulated polluted marine atmosphere. The results indicated that the high-strength steel without Nb and Sb was badly damaged by corrosion and very susceptive to SCC in a SO2-containing marine atmosphere, which was characterized by high corrosion rate and a large number of potential initiation sites for SCC cracks. By contrast, Nb addition affected little on atmospheric corrosion rate, but weakened the role of hydrogen at the boundaries of prior austenite grain and lath bainite. Sb addition significantly reduced the corrosion rate by optimizing the properties of rust layer through its synergy with Cu, and inhibited the localized anodic dissolution and acidification effect underneath. Thus, the combined addition of Nb and Sb increased the resistance to atmospheric corrosion and substantially inhibited SCC initiation.

In this study, the effect of Co addition on the microstructure and creep rupture properties of 9Cr-1.8W-xCo weld metals is investigated. Herein, stick electrodes were fabricated for producing weld metals with 0.5, 1.0, and 1.5 wt pct Co and without Co, which were then processed by post-welding heat treatment at 760 °C for 4 hours. The microstructures of weld metals, including the prior austenite grain size, tempered martensite lath size, delta ferrite content, and precipitate quantity and size, were characterized via optical microscopy, scanning and transmission electron microscopy, and electron backscattered diffraction. The precipitates were identified by X-ray diffraction after they were extracted from the matrix. The Curie temperature was determined via differential scanning calorimetry, which allowed the evaluation of diffusion rate as a function of Co content. The weld metals were mechanically characterized by creep rupture tests conducted at 675 °C under 150 MPa and by hardness and tensile tests conducted at room temperature as well as Charpy impact tests conducted at various temperatures from − 40 °C to 60 °C. The results obtained herein suggest that the addition of Co significantly increases the creep rupture time of 9Cr-1.8W steel (by approximately 20 times than that of Co-free steel), reduces the prior austenite grain size and tempered martensite lath size, hinders the formation of detrimental delta ferrite, and promotes the formation of precipitates with slightly larger size. Herein, the highest creep resistance from the viewpoint of creep rupture time was obtained for 9Cr-1.8W weld metal with 1.5 wt pct Co. Furthermore, the results suggest that a finer and more stable substructure is required for improving the creep resistance.