690 MPa级海洋平台用特厚齿条钢的回火稳定性

Tempering Stability of a 690 MPa Grade Ultra-heavy Rack Steel for Offshore Platform

研究了长时间回火(2,4和8 h)对海洋平台用齿条钢A514 GrQ显微组织及力学性能的影响.结果表明,回火过程中,淬火态马氏体板条组织发生回复,C,Cr,Mo和V等元素扩散,析出MC,M_(2)C和M_(23)C_(6)型碳化物,回火2 h后碳化物类型趋于稳定,并发生Ostwald熟化现象;随着回火时间的增加,实验钢的抗拉强度由淬火态的1100 MPa逐渐降低到回火8 h时的813 MPa,屈服强度由淬火态的931 MPa逐渐降低至回火8 h时的725 MPa;淬火态实验钢回火2 h后,-60℃冲击功由淬火态的187 J增加至238 J,继续增加回火时间,冲击功趋于稳定,约为245 J;延伸率由淬火态的15.2%增加至回火2 h时的16.7%,并且随着回火时间的增加,升高至回火8 h时的19.5%.实验钢在600℃回火2~8 h过程中,具有良好的回火稳定性和强韧性匹配,最佳回火时间为2 h.

The effects of long-term tempering(2 h,4 h and 8 h)on microstructure and mechanical property of rack steel A514 GrQ for offshore platforms were studied.It is found that during the tempering process,the quenched martensite lath structure recovered,C,Cr,Mo and V elements diffused,and MC,M_(2)C,and M_(23)C_(6) type carbides were precipitated in the martensite matrix.After tempering for 2 h,the carbides tended to be stable and the Ostwald ripening occurred.With the increase of tempering time from 0 h to 8 h,tensile strength of the tested steel decreased from 1100 MPa to 813 MPa and yield strength decreased from 931 MPa to 725 MPa.The impact energy at-60℃increased from 187 J of the as-quenched steel to 238 J after 2 h tempering and then it tended to be stable with a value about 245 J for further tempering.The elongation increased from 15.2%of the as-quenched steel to 16.7%after 2 h tempering and further increased up to 19.5%for 8 h tempering.These results indicate that in the process of tempering at 600℃for 2-8 h,the experimental steel A514 GrQ can perform the good tempering stability and the excellent combination of strength and toughness,associated with the best tempering time of 2 h.