Effect of Tempering Temperature on Microstructure and Sulfide Stress Cracking of 125 Ksi Grade Casing Steel

The influence of tempering temperature on the microstructure of 0.5Cr0.4W steels was investigated by scanning electron microscope, and the roles of grain boundary character, dislocation, and Taylor factor in sulfide stress cracking (SSC) resistance were interpreted using the election backscattered diffraction technique. The 0.5Cr0.4W steels tempered at 690 degrees C, 700 degrees C, and 715 degrees C all showed tempered martensites. The specimen tempered at 715 degrees C exhibited a higher critical stress intensity factor (K-ISSC) of 34.58 MPa center dot m(0.5), but the yield strength of 800 MPa did not meet the criterion of 125 ksi (862 MPa) grade. When the specimen was tempered at 690 degrees C, the yield strength reached 960 MPa and the K-ISSC was only 21.36 MPa center dot m(0.5), displaying poorer SSC resistance. The 0.5Cr0.4W steel tempered at 700 degrees C showed a good combination of yield strength (887 MPa) and SSC resistance (K-ISSC: 31.16 MPa center dot m(0.5)). When increasing the tempering temperature, the local average misorientation and Taylor factor of the 0.5Cr0.4W steels were decreased. The reduced dislocation density, and greater number of grains amenable to slippage, produced less hydrogen transport and a lower crack sensitivity. The SSC resistance was, thus, increased, owing to the minor damage to hydrogen aggregation. Therefore, 700 degrees C is a suitable tempering temperature for 0.5Cr0.4W casing steel.