Effect of Cryogenic Treatment on Microstructure, Mechanical Properties and Distortion of Carburized Gear Steels

Abstract

The effects of cryogenic treatment and low temperature tempering on the microstructure, mechanical properties and distortion of the 20Cr2Ni4A and 17Cr2Ni2MoVNb carburized gear steels were investigated. The results showed that the case hardness of the experimental steels was increased after the cryogenic treatment, due to the decrease of the retained austenite content and the precipitation of the tiny carbides. The wear resistance of the two steels after cryogenic treatment was improved, although the wear mechanisms were different for 17Cr2Ni2MoVNb and 20Cr2Ni4A steels. The distortion of the Navy C-ring specimens underwent shrinkage before expansion during the cryogenic process, and the distortion of 17Cr2Ni2MoVNb steel was smaller than that of 20Cr2Ni4A steel.

Cryogenic Treatment of Martensitic Steels: Microstructural Fundamentals and Implications for Mechanical Properties and Wear and Corrosion Performance

“…CT increased hardness by 0.6 HRC for 18NiCrMo5 steel [235], but there was almost no effect for gear wheels made of 18CrNiMo7-6 steel treated at −30, −40, −80, or −196 • C [226]. There were also negligible hardness increments observed for AISI 8620 steel (CT at −40 • C for 1 h) [231] and for 17Cr2Ni2MoVNb and 20Cr2Ni4A steels (CT at −196 • C for 1 h) [230].…”

“…In SNCM 415, a hardness increase of almost 200 HV was observed due to CT at −85 • C for 24 h [225]. For 17Cr2Ni2MoVNb, 21NiCrMo2, and 20Cr2Ni4A steels, there was a hardness increase of almost 40, 80, and 120 HV1, respectively, after CT at −80, −150, or −196 • C (each of them for 1 h) [229,230] or after CT at −120 • C for 2 h [234]. The hardness increase due to cryogenic treatments is a result of a 30 to 50% reduction in retained austenite as well as more extensive precipitation of fine carbides due to cryogenic treatment [229].…”

“…Even though they contain more than 20 vol.% of RA after conventional quenching, the retained austenite can be reduced to one-half [47,225,227,229] or one-fourth [180] by applying sequence A. A typical example is the work by Yan et al [229], who reported retained austenite reduction from 18.15% (CHT) to 12.92, 10.73, and 9.45% for CT at −80, −150, and −196 • C. On the other hand, the effect of cryogenic treatment on retained austenite reduction is suppressed when steels are pre-tempered prior to CT (sequences E or F), which is due to the stabilisation of retained austenite [230,231].…”

“…The principal explanation for the CT-controlled hardness increase is that cryogenic treatment reduces the retained austenite amount. This reduction is greater for Ni-containing steels [226,229,230] than for steels without Ni [225]. Tempering prior to CT thermally stabilises retained austenite [126,324].…”

Analyze the Wear Mechanism of the Longwall Shearer Haulage System

“…Hard oxides cause micro-cutting. In this sample, there is a set of phenomena such as adhesive, abrasive wear, oxidation wear and fatigue [40,55].…”