Distortion in quenching an AISI 4140 C-ring – Predictions and experiments

Abstract

Heat treatment processes are used to enhance the material properties of a wide range of mechanical steel components, according to their final application. Quenching is a common step in these heat treatments, involving the fast cooling of previously austenitized parts and leading to a phase transformation from austenite to hard martensite in the material. Quenching commonly causes a geometric distortion in the parts, associated with the thermal contraction and with the change in the mechanical and geometrical properties of austenite and martensite. It is of importance to predict these distortions, so that one can design corrective post-heat treatment shape corrections or include them in the pre-heat treatment part dimensions, thus leading to a final part with adequate shape and dimensions. This study presents the results of a finite element (FE) simulation of the quenching of an AISI 4140 steel C-ring in oil, covering the analysis of the distortion caused by both thermal contraction and phase transformation. Furthermore, the distortion behavior during the cooling stage is analyzed, as well as the hardness and martensite volume fractions. Experiments were also conducted in order to obtain the geometric distortion, the microstructures and hardness of the C-rings. The FE modeling results are in good agreement with the experimental values and, to the knowledge of the authors, this is the first time that such an agreement has been obtained for the distortion caused by quenching of C-rings. The design of new products and quenching processes should consider the studied aspects, and may also be assisted by the methodology applied to this work.

Introduction

Heat treatment is used to improve some of the mechanical properties of steel components, and commonly involves a quenching step which may cause undesired geometric distortions in the processed parts. The dimensional accuracy of these parts is affected and leads to production and economical losses. An example of this situation is the production of rolled and heat treated rings with large diameters and small thickness, where quenching causes out-of-roundness of rings. Various factors including phase transformation of steel, retained austenite, quench media, severity and uniformity and process selection may influence the final dimension of a quenched part [1], [2], [3].

It is frequently possible to minimize the quench distortion through the control of the heat transfer coefficient around parts such as rings [4]. However, in this case a variety of ring sizes and materials quenched in the same production line makes this approach not practical. As a result, distorted rings must undergo a radial compression or expansion step, based on prior industrial experience, in order to correct the out-of-roundness shape, increasing production time and costs [5].

One way of circumventing the problems connected to the distortion of parts caused by heat treatments is to take this distortion into consideration already in the design and manufacturing steps. If the new shape of a part after heat treatments were accurately predicted, then compensation measures could be included in the design stage prior to manufacturing. Another possibility for the correction of the heat treatment distortions would be to design adequate post-heat treatment forming operations that would eliminate the distortions. Both approaches represent fundamental design procedures in order to obtain a final adequate part, and depend on the possibility of predicting accurately the distortions caused by the heat treatments.

A finite element simulation of the quenching of carburized 4140 steel rings (75 mm OD) has been performed [6], discussing the origin of the quench distortion, considering carbon concentration, temperature evolution and volume fraction of the various metallic phases. The Navy C type test is frequently employed [7] for evaluating the propensity for quench distortion in several materials. This test has also been used to investigate the influence of the chemical composition on the hardenability and quench distortion of automotive [8], [9]. The complexity of the numerical simulation of such heat treatment distortions was studied [10], [11], and the results were qualitatively, but not quantitatively, similar to the experimental ones. Another use of the Navy C-ring test was the evaluation and definition of a mathematical model for the quenching of small stainless rings [12]. Numerical simulations and experimental tests for the quenching of cast steel C-rings were conducted [13], but did not lead to an adequate correlation for the experimental and predicted magnitudes of the distortions. Experimental quenching of C-ring specimens for different materials (including 4140 steel) have been conducted [14], [15], and the resulting gap opening and outside diameter of the C-rings have been reported. The simulation of the quenching of C-rings based on 4140 steel material data has been performed, and differences between the results and those reported in the literature were reported [16]. In addition, a good correlation between the simulated and experimental results for quench distortions depends critically on the knowledge of the properties of the materials during their cooling [1].

No satisfactory comparison between numerical simulations and experimental results on the distortion and microstructural and hardness distribution could be found in the literature for quenched 4140 C-rings. The objective of the present paper is to present such an analysis. In this study, a good agreement was found between the experimental and simulation results. It is expected that the present methodology will contribute to the design of new parts submitted to heat treatment, so that their final shape and dimensions after heat treatment and eventual corrective forming procedures is the desired one.