Spheroidization Heat Treatment Conditions with Data Analysis in Medium Carbon Cr-Mo Steel for Ultra High Strength Cold Heading

Abstract

The degree to which parameters affect the spheroidization heat treatment of steel was calculated by setting the spheroidization heat treatment conditions of Cr-Mo steel and using data analysis such as S/N ratio and ANOVA. After analyzing the transformation temperatures of the steel, A<sub>c1</sub> and A<sub>c3</sub> , using a DSC, the conditions were set accordingly. The surface hardness was measured for the conditions and used as an evaluation index. The correlation was analyzed by comparing the spheroidized volume fraction and the surface hardness, and the Pearson correlation coefficient was -0.88, proving that a correlation existed between the two values. Using S/N ratio and ANOVA, the degree to which each control parameter affects the decrease in the surface hardness was analyzed, qualitatively and quantitatively. For the S/N ratio, priority affecting the surface hardness for each control parameter was analyzed. The 1<sup>st</sup> heating temperature was found to have a more preferential effect on the surface hardness than the 1<sup>st</sup> heating time and the 2<sup>nd</sup> heating temperature. Using ANOVA, the 1<sup>st</sup> heating temperature was determined to be a very significant factor with the greatest influence, contributing 73.2% to the surface hardness. Intercritical annealing is a suitable spheroidization heat treatment condition, so if the surface hardness of the steel needs to be reduced using Intercritical annealing, the 1<sup>st</sup> heating temperature and time should be designed as the priority.