Determination of Critical Transformation Temperatures for the Optimisation of Spring Steel Heat Treatment Processes

Abstract

Bogie spring performance can be improved by using the exact heat treatment process parameters. The purpose of the study is to determine the critical transformation temperatures and investigate the effect of the cooling rates on microstructural and mechanical properties. The precise determination of the required cooling rates for the particular grade of steel is important in order to optimise the heat treatment process of heavy-duty compression helical spring manufacturing. A traditional heat treatment system for the manufacture of hot coiled springs requires heating the steel to homogenize austenite; then, it is decomposed to martensite by rapid cooling. By analyzing the transition properties by heating and differing cooling rates, this analysis examines the thermal behaviour of high strength spring steel. Using the dilatometer and differential scanning calorimeter, scanning electron microscope, optical microscope, and hardness checking, critical transition temperatures and cooling rates of three springs steels were measured. Although the thermal transformation of materials has been researched for decades using dilatometers, not all materials have been characterized. The research offers insights into the critical transformation temperatures for the defined grades of spring steel and the role of cooling rates in the material’s properties. Mechanical properties are influenced by the transition data obtained from the dilatometric analysis.

Using dynamic materials modelling approach to establish critical parameters for hot coiling of spring steels

“…Power dissipation (Figure 5) and instability maps (Figure 6) were developed from the experimental data to understand the response of the three spring steels to imposed deformation temperature and strain rates. Selected hot working temperatures were guided by previous work [35,36,37]. From the results, it was established the spring steels all exhibited generally low power dissipation efficiency ranging between 7-30 % (Figure 5) indicating high deformation resistance.…”

“…The soaking time for both austenitising and tempering plays an important role in the outcome of the microstructure and mechanical properties of spring steels [10]. Consequently, some retained austenite, bainite or pearlite may be found in the microstructure spring steel [11,12].…”