FEATURE ADVANCED MATERIALS & PROCESSES | APRIL 2025 52 A dilatometry test is an effective method to characterize the phase transformation kinetics for heat treatment processing of steels. Two commonly used machines for dilatometry testing include the quench dilatometer and Gleeble system. A quench dilatometer measures the axial length change of a cylinder during the heating and cooling cycle, while the Gleeble measures the diameter change of the cylinder in most cases. TEST SETUP It is important to select a proper cylinder size and design the heating and cooling rates to control the gradients of temperature and phase transformations in the sample. For most tests, the diameter of the cylindrical sample is between 3 and 10 mm. Because the temperature gradient in the sample is unavoidable, especially during a fast-quenching step, the collected strain data contains inaccuracy. It is important to understand the root causes of the inaccuracy in the measured dilatometry data for the purpose of optimizing a test matrix, including the selections of sample size and quenching rates. Figure 1 shows an experimental dilatometry displacement curve for 5250 steel using a quench dilatometer. The circled DATA INACCURACY INVESTIGATION IN DILATOMETRY STRAIN DATA FOR PHASE TRANSFORMATION CHARACTERIZATION USING COMPUTER MODELING This study uses computer modeling to look at the root causes of inaccuracies in measured dilatometry displacement data using a quench dilatometer. Zhichao (Charlie) Li* DANTE Solutions Inc., Cleveland regions A and B are commonly seen in dilatometry tests using high quenching rates or improper sample sizes[1,2]. This article focuses on investigating the root causes of the inaccuracy in the dilatometry strain data with the help of computer modeling of dilatometry tests using a quench dilatometer. The commercial heat treatment software, DANTE, is used to model the temperature, phase transformations, in-process stress, and sample size change during the heating and cooling steps. COMPUTER MODELING As shown in Fig. 2, the selected dilatometry sample is a cylinder with 5-mm diameter and 10-mm length. The sample is made of AISI 4140 hot rolled bar stock, and a mixture of ferrite and pearlite is the initial sample microstructure. The axial displacement is measured for characterization of the material properties, including the coefficient of thermal expansion (CTE) of different phases, phase transformation kinetics, and transformation strains. Two dilatometry curves are calculated in Fig. 2, an ideal quenching without temperature gradient in the sample, and a fast quenching with 2000 W/(m2K) heat transfer coefficient (HTC) and 20°C nitrogen gas. An axi- symmetric cross-section of the sample is modeled with the finite element meshing shown in Fig. 2. There are noticeable differences between the two calculated curves, as highlighted in Region A and Region B. The calculat- *Member of ASM International Fig. 1 — An experimental dilatometry curve for 5250 steel using quench dilatometer. Fig. 2 — Dilatometry sample size and simulated strain change in terms of sample temperature. 8
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