Fracture Behavior of High-Nitrogen Austenitic Stainless Steel Under Continuous Cooling: Physical Simulation of Free-Surf
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Heavy forgings, such as heavy rollers,[1] heavy rotors,[2,3] and reactor pressure vessels,[4,5] are widely used in metallurgical and energy industries; however, these forgings are difficult to manufacture. One difficulty is the need to prevent the occurrence of free-surface cracking during hot forging. When a crack appears on the free surface of the workpiece, the forging process is disrupted and may even be terminated.[6] Therefore, clarification of the cracking mechanism and associated influencing factors would have important industrial implications. Free-surface cracking of heavy forgings has attracted considerable attention in the past. The effects of
ZHENHUA WANG is with the School of Mechanical Engineering, Yanshan University, Qinhuangdao 066004, P.R. China, with the State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, and also with the Hebei Iron & Steel Technology Research Institute, Shijiazhuang 050023, P.R. China. Contact e-mail: [email protected] HONGPENG XUE is with the School of Mechanical Engineering, Yanshan University. WANTANG FU is with the State Key Laboratory of Metastable Materials Science and Technology, Yanshan University. Manuscript submitted October 25, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
inclusions,[7,8] second phases,[9,10] grain size,[6,11] and strain rate and temperature[12] on cracking have been thoroughly investigated mainly using hot tensile and compression tests with deformation performed at a constant temperature. However, these conditions differ from those encountered during the actual production of heavy forgings. Heavy forgings are subjected to very low strain rates during hot forging, especially during the upsetting stage, which generally lasts a few to tens of minutes.[13] Free-surface cracks nucleate and propagate under the continuous cooling of this process. This issue is not encountered for small- and medium-sized forgings. Developments in the energy industry are likely to lead to further increases in forging size[4,14]; therefore, fracture behavior under continuous cooling is becoming increasingly important and requires further investigation. 18Mn18Cr0.6N steel was selected as a model material in this study. It was melted in a vacuum electric furnace and then electroslag remelted. The chemical composition of the steel was (weight percent): 0.084 C, 17.9 Mn, 18.06 Cr, 0.62 N, 0.46 Si, 0.2 Ni, 0.009 P, 0.002 S, and balance Fe. Small slabs were cut from the ingot and rolled at 1373 K (1100 C). The total true strain during rolling was ~ 2. The rolled slabs were heat treated at 1473 K (1200 C) for 3 hours to obtain an average grain size of 305 lm. Hot tensile tests were conducted using a Gleeble-3800 simulator. First, a specimen (6-mm diameter 9 120-mm length, 12-mm gage length) was preheated to 1473 K (1200 C) and then tension tested at a strain rate of 0.001 s 1 to fracture with a cooling rate of 0.4 Ks 1. This specimen is referred to as T1473-1363. The fracture temperature was 1363 K (1090 C). For comparison, two additional s
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