MODELING OF INDUCTION HARDENING PROCESSES PART 2: QUENCHING AND HARDENING Dr. Jiankun Yuan Prof. Yiming (Kevin) Rong Acknowledgement: This project is partially supported by Delphi and CHTE at WPI. Dr. Q. Lu was involved in the early work of the project. http://me.wpi.edu/~camlab Objectives • To develop a numerical modeling system for analyzing quench cooling and hardening processes based on temperature field data after induction heating. • To provide temperature distribution in workpiece at any time in a quenching process • To provide continuous cooling curves (CCC) of any location in workpiece, for phase transformation analysis. • To build an algorithm to analyze phase transformation in quench cooling processes, based on time-temperature-transformation (TTT) and CCC curves. •To provide time traces of volumetric content of martensite, pearlite and bainite formed in cooling. • To formulate a relationship between martensite content and hardness values, and to provide hardness patterns formed after quenching process. • To investigate key parameters (input AC power, frequency and gap between coil and workpiece) effects on final hardening patterns. Principle: Phase Transformation Phase transformation kinetics from austenite to pearlite, bainite and martensite Koistinen-Marburger model for martensite content determination TTT diagram f m (1 f p fb )(1 er ( M s T ) ) r= 0.01-0.015 (fs,ts) Avrami model for fp, fb determination in isothermal (fe,te) transformation ktn f 1 e ln(1 f s ) Generally ln n(T ) ln(1 f e ) fs=0.5%, fe=99.5% TTP curve t ln s t ti e Ms ln(1 f s ) k (T ) t sn (T ) For continuous cooling , fp, fb can be determined using following expressions fi (kntn 1ekt )i ti n •Ms: Martensite start temperature N f f i •Martensite can only be formed from austenite i 1 after WP temperature lower than Ms Principle: Hardeness Analysis Principle: Relationship between martensite content and hardness fm HRC Aim of hardening 0.5 47.2 analysis 0.8 50.3 0.9 53.7 0.95 56.3 0.99 58.8 0.47% HRC 82 .47 ( f m ) 2 99 .21( f m ) 76 .187 General expression: HRC a( f m ) 2 b( f m ) c Page 144, <<Steel and its Heat treatment>>, Karl-Erik Thelning Coefficients a,b,c varying with carbon content For AISI 1070, 0.7% carbon, a=80.91,b=97,c=81.61 Case Study:Temperature Field Variation in Water Quenching Process t=0.5s Total quenching time t=2s tq = 40s f=9600Hz s=1.27mm J=1.256e6 A/m2 t=8s t=40s Case Study: Cooling Curves and Hardening Pattern Surface points Inside points along contour line T=8150C Material: Carbon Steel, AISI 1070 Hardness pattern form Automotive parts from numerical simulation Delphi Inc., Sandusky,Ohio Case Study: Gap Effect - Hardening Pattern Variation with Tolerance Tolerance= - 0.0025” Tolerance= 0” S=1.2065mm S=1.27mm f=9600Hz f=9600Hz J=1.265e6 J=1.265e6 Tolerance= + 0.0025” Fig. Hardening depth variation with gap S=1.3335mm between coil and workpiece under three f=9600Hz different frequencies. J=1.265e6 • Hardening depth decreases with air gap distance Power Effect - Hardening Pattern Variation with Coil AC Current Density s 1.27mm s 1.27mm J 7 106 A / m 2 J 1.256 107 A / m 2 f 9600Hz f 9600Hz th 7s th 7 s t q 40s tq 40s (a) J 7 10 6 A / m 2 (b) J 1.256 10 7 A / m 2 s 1.27mm s 1.27mm J 2.25 107 A / m 2 J 3 107 A / m 2 f 9600Hz f 9600Hz th 7s th 7 s Fig. Hardening depth variation with input t q 40s t q 40s current density with f=9600Hz, s=1.27mm • Case depth increase with input AC power (c) J 2.25 10 7 A / m 2 (d ) J 3 10 7 A / m 2 Hardening Pattern Variation with Input AC Frequency s 1.27mm s 1.27mm J 1.256 107 A / m 2 J 1.256 107 A / m 2 f 5000Hz f 9600Hz th 7 s th 7 s tq 40s tq 40s (a) f=5000Hz (b) f=9600Hz s 1.27mm Fig. Hardening depth variation with input J 1.256 10 A / m 7 2 current frequency with J=1.256e7 (A/m2), f 15000Hz s=1.27mm th 7 s tq 40s •Case depth decrease with input AC frequency. (c) f=15000Hz Summary • A quenching and hardening modeling system was developed with the following capabilities. (1) Provide workpiece temperature distribution at any time. (2) Provide cooling curve data of any location in workpiece. (3) Simulate the phase transformation process and predict volume fraction of Pearlite, Bainite, Martensite formed in cooling process. (4) Provide desired hardness pattern through proper simulation of coil design and optimum combination of control parameters. (5) Investigate parameters effects on final hardness pattern, including gap effect, AC frequency effect and current density effect. • Applied the developed system to investigate the hardening process on a complex surface of an automotive spindle.
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