Date Published: 06/22/2022
Abstract:
Work rolls used in the rolling of steel billets are generally made of high-hardenability chromium alloy steel and must be heat treated prior to entering service to ensure a sufficiently hard working surface. In combination with a hard surface, the roll should also possess superior resistance to wear and thermal shock. [1] Compared to other surface hardening methods, induction hardening (IH) has become a popular process for hardening steel work rolls. However, designing the critical IH process parameters can be challenging, given the large temperature gradients induced in the roll.
The large temperature gradients encountered during induction heating, and the subsequent liquid quench, can present themselves radially and axially during the scanning induction hardening process of a large work roll. Consequently, small variations in processing parameters can have significant effects on the microstructure obtained and the in-process and residual stresses. Many IH processing parameters can have a significant effect on the product’s in-service performance and include, but are not limited to, the preheat temperature, frequency and power supplied to the inductor, cooling power of the quenchant, relative inductor scanning rate, inductor or quench head dwells, and the environmental temperature. [1]
Surface treatments, in combination with increasing surface hardness, aim to induce residual surface compressive stresses. Residual stress has long been known to significantly influence the fatigue and wear performance of steel work rolls. High levels of surface compressive stresses are desirable to arrest crack formation and offset any tensile loads experienced by the roll in service. However, the surface compression must be balanced by tensile stresses, which in an induction hardened work roll, generally occur just below the hardened layer. [2 – 5] Given the location of the tensile stress peak subsurface, and the Hertzian stress distribution induced from rolling contact, a properly designed case depth and residual stress profile is critical in ensuring the work roll is able to withstand the harsh conditions during operation.
Given the breadth of possible process parameter combinations available for any given induction hardening process, and the significant time and cost associated with physical, experiment-driven trial-and-error approaches, computer simulation is well suited for the task of process parameter evaluation and determination. The following examples utilize the commercially available heat treatment simulation software DANTE to examine how heat treatment simulation can be used to evaluate and determine appropriate process parameters for the induction hardening of a steel work roll to meet case depth and hardness requirements, while ensuring a low probability of developing quench cracks. Distortion and residual stresses are also predicted by DANTE, which are critical for evaluating product performance in-service. [5 – 6]
With computer hardware and software becoming available to a wider group of industry analysts and experts, in-service performance and fatigue modeling of components prior to physical testing are being utilized across many industries to discover design problems prior to any physical part being manufactured. The importance of residual stress on in-service and fatigue performance has been previously mentioned, but if the post-heat treatment models are to accurately represent reality, then the residual stress state from heat treatment must be considered as the initial condition for these subsequent simulations. [7 – 8] Residual stress is also an important consideration when performing any post-heat treatment or mid-life rework grinding operations, as the removal of material will cause a rebalancing of the residual stresses. If the tensile field is too close to the surface after the initial surface hardening, grinding may bring the tensile field within the Hertzian contact stress field and can cause premature failure of the roll in-service. [4]
Author: Justin Sims, Jason Meyer, and Zhichao Li
Tags: Induction, Steel, Work Roll, and Simulation
Process Used: Induction Hardening
Other Topics: