Effect of Zr Microalloying on Sulfide Stress Cracking Resistance in 125ksi Grade Oil Country Tubular Goods Steel

The effect of Zr addition on the sulfide stress cracking (SSC) resistance has been investigated for two experimental steels. Both experimental steels are undergone the identical heat treatment; however, experimental results disclosed that the strength and toughness of the Zr-steel are improved simultaneously (Rσ = 913 MPa, AKV = 161 ± 9 J), and the Zr-steel passed the experiment of NACE-A. The strength (Rσ = 868 MPa), toughness (AKV = 86 ± 3 J) and sulfide stress corrosion resistance (the fracture time of NACE-A is 72 h) of the Basic steel are relative worse. The mechanical property and SSC resistance are affected not only by the microstructures, but also the prior austenite grain size and the second phase particle. Besides, the average size of the prior austenite grain of the Zr-steel (6.47 ± 0.2 μm) is consistently smaller than that of the Basic steel (7.67 ± 0.2 μm), so as the final tempered martensite structure. The size of prior austenite grain is related to the high volume fraction of carbides and nitrides, working to pin the austenite gain. Moreover, the strengthening mechanism for SSC resistance by inclusion is discussed via fracture mechanics. The second particles caused by Zr is fine, which leads to the improvement in performance.