A Study of Temper Brittleness and Overall Immersion Quenching and Tempering Treatment of Cr5M Back-up roll

The heat treatment experiment was conducted on Cr5M steel to investigate the relationship between temper brittleness and hardness and the effect of microstructure on temper brittleness. Based on these investigations, an overall immersion treatment process meeting the hardness requirement of Cr5M back-up roll was proposed, and the overall immersion quenching and tempering treatment effect of Cr5M steel support rollers was explored. It is found that Cr5M steel exhibits a certain degree of temper brittleness characteristics at the temper temperature range of 300–500 ℃. The lowest impact absorbed energy is ascertained to be 7.6 J for the sample tempered at 400 ℃. The retained austenite (Ar) in the quenched Cr5M steel begins to decompose at the temper temperature of about 250 ℃, and the decomposition end temperature is confirmed to be about 400 ℃. The decomposition of soft phase Ar plays a key role on the temper brittleness of Cr5M steel. The overall immersion treatment (quenching at 960 ℃ and tempering at 615 ℃) can replace the differential heat treatment or surface quenching and tempering treatment. The excellent comprehensive properties are obtained for the overall immersion quenched and tempered Cr5M back-up roll. The uniform surface hardness is achieved for the roll, the value of maximum and minimum Shore hardness (HSD) difference is only 0.7 HSD, and the average hardness is 40.1 HSD. The working ling (quenching layer) depth of the roll is more than 15 cm. The microstructure of working ling is consisted of tempered sorbite and a small amount of ferrite, and its hardness drop is only 2.1 HSD.