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The microstructures and mechanical properties of X80 pipeline steels produced by both novel ultra fast cooling and conventional-accelerated continuous cooling modes are investigated. Results showed that different levels of Mo addition had a remarkable effect on the microstructures and mechanical properties of the investigated pipeline steels. The proeutectoid ferrite and pearlite formation is inhibited in the high-Mo steel and acicular ferrite is obtained over a wide range of cooling rates, whereas the dominant acicular ferrite microstructure can only be obtained when the cooling rates reach up to 5 C s-1. Very similar microstructures and mechanical properties are obtained in the low-Mo steel produced with ultra fast cooling and in the high-Mo steel produced by the conventional-accelerated continuous cooling. It was proved by simulation and industrial trials that high-strength low-alloy steels such as pipeline steels, can be produced using the novel ultra fast cooling which also reduce alloy cost. Very similar microstructures and mechanical properties are obtained in the low-Mo steel produced with ultra fast cooling and in the high-Mo steel produced by the conventional-accelerated continuous cooling. It is proved by simulation and industrial trials that high-strength, low-alloy steels such as pipeline steels can be produced using the novel ultra fast cooling which also reduces alloy cost.

The effect of tempering temperature, within the range of 400 to 700°C, on the microstructure and hardness of two super-bainitic steels, one as the control parent sample and the other with added Co & Al was investigated. Post-tempering examinations of the super-bainitic samples showed that low temperature tempering cycles (400–500°C) resulted in carbides formation, and some increases in the hardness possibly due to precipitation strengthening in the Co & Al contained steel. Once the tempering temperature increased to 600°C, the hardness plummeted in both steels due to the concurrent coarsening of the bainitic ferrite plates and more precipitation of carbides. At the higher tempering temperature of 700°C, further reduction in the hardness occurred because of the accelerated recovery of ferrite and spheroidization of carbides. This work clearly showed that the super-bainitic steel containing Co & Al had a superior tempering resistance particularly at low tempering temperatures (<500°C) due to reduced carbide precipitation in the presence of Co & Al.

The effect of a deep cryogenic treatment on the microstructure of a super-bainitic steel was investigated. It was shown that quenching the super-bainitc steel in –196°C liquid nitrogen resulted in the transformation of retained austenite to two phases: ~20 nm thick martensite films and some nano carbides with a ~25 nm diameter. Some refinement of the retained austenite occurred, due to formation of fine martensite laths within the retained austenite. The evolution of these new phases resulted in an increase in the average hardness of the super-bainitic steel from 641 to ~670 HV1.

The tempering resistance and stability of retained austenite in super-bainitic and quenching-partitioning martensitic steels were investigated over the temperature range of 400 to 700 °C. The X-ray diffraction analysis and hardness tests showed that the quenching-partitioning-martensitic steel contained a considerable amount of retained austenite (26.6 vol%) and had a relatively high hardness up to 556 HV1 after tempering at about 600 °C. In contrast, the fraction of retained austenite and hardness of super-bainitic steel were considerably lower (24.5 vol% and 385 HV1) after the same tempering cycle. The work also showed that the quenching-partitioning steel had a higher tempering stability, probably, due to the higher fraction of carbon-rich retained austenite.

The effect of tempering temperature, within the range of 400 to 700°C, on the microstructure and hardness of two super-bainitic steels, one as the control parent sample and the other with added Co & Al was investigated. Post-tempering examinations of the super-bainitic samples showed that low temperature tempering cycles (400–500°C) resulted in carbides formation, and some increases in the hardness possibly due to precipitation strengthening in the Co & Al contained steel. Once the tempering temperature increased to 600°C, the hardness plummeted in both steels due to the concurrent coarsening of the bainitic ferrite plates and more precipitation of carbides. At the higher tempering temperature of 700°C, further reduction in the hardness occurred because of the accelerated recovery of ferrite and spheroidization of carbides. This work clearly showed that the super-bainitic steel containing Co & Al had a superior tempering resistance particularly at low tempering temperatures (<500°C) due to reduced carbide precipitation in the presence of Co & Al.

The effect of a deep cryogenic treatment on the microstructure of a super-bainitic steel was investigated. It was shown that quenching the super-bainitc steel in –196°C liquid nitrogen resulted in the transformation of retained austenite to two phases: ~20 nm thick martensite films and some nano carbides with a ~25 nm diameter. Some refinement of the retained austenite occurred, due to formation of fine martensite laths within the retained austenite. The evolution of these new phases resulted in an increase in the average hardness of the super-bainitic steel from 641 to ~670 HV1.

The tempering resistance and stability of retained austenite in super-bainitic and quenching-partitioning martensitic steels were investigated over the temperature range of 400 to 700 °C. The X-ray diffraction analysis and hardness tests showed that the quenching-partitioning-martensitic steel contained a considerable amount of retained austenite (26.6 vol%) and had a relatively high hardness up to 556 HV1 after tempering at about 600 °C. In contrast, the fraction of retained austenite and hardness of super-bainitic steel were considerably lower (24.5 vol% and 385 HV1) after the same tempering cycle. The work also showed that the quenching-partitioning steel had a higher tempering stability, probably, due to the higher fraction of carbon-rich retained austenite.