The Effects of Rust on the Gas Carburization of AISI 8620 Steel by Xiaolan Wang A Thesis Submitted to the Faculty of the WORCESTER POLYTECHNIC INSTITUTE in partial fulfillment of the requirements for the Degree of Master of Science in Material Science July 2008 Approved: ___________________ Richard D. Sisson, Jr. Director of Manufacturing and Materials Engineering George F. Fuller Professor Abstract The effects of rust on the carburization behavior of AISI 8620 steel have been experimentally investigated. AISI 8620 steel samples were subjected to a humid environment for time of 1 day to 30 days. After the exposure, a part of the samples was cleaned by acid cleaning. Both cleaned and non-cleaned samples have been carburized, followed by quenching in mineral oil, and then tempered. To determine the effect of rust on gas carburizing, weight gained by the parts and the surface hardness were measured. Surface carbon concentration was also measured using mass spectrometry. Carbon flux and mass transfer coefficient have been calculated. The results show that acid cleaning removes the rust layer effectively. Acid cleaned samples displayed the same response to carburization as clean parts. Rusted parts had a lower carbon uptake as well as lower surface carbon concentration. The surface hardness (Rc) did not show a significant difference between the heavily rusted sample and clean sample. It has been observed that the carbon flux and mass transfer coefficient are smaller due to rust layer for the heavily rusted samples. These results are discussed in terms of the effects of carbon mass transfer on the steel surface and the resulting mass transfer coefficient. 2 Acknowledgements This project was sponsored by WPI’s Center for Heat Treating Excellence. I would like to thank my advisor Professor Richard D. Sisson, Jr. for providing me the opportunity to work on this project and for his help, encouragement, and advice throughout this project as well as others to allow me to be where I am today. I would like to thank Prof. Mohammed Maniruzzaman for all of his invaluable assistance and suggestions throughout this study. I would like to thank Prof. Jianyu Liang to be my thesis committee. I would also like to thank Rita Shilansky for all of her time and support throughout this project. I thank Olga Karabelchtchikova of Caterpillar Inc. for her help and suggestions. I thank Bodycote, Worcester plant for doing the heat treatment. I thank Dr. Gang Wang for doing the modeling. I also like to thank Dr. Li Boquan for the help with all the lab work. Lastly, I thank my friends and family for all of their help to get me where I am. Worcester, MA July 2008 3 Table of Contents Abstract………………………………………………………………...2 Acknowledgements………………………………………………….…3 Chapter 1 Introduction…………………………………………………5 Chapter 2 Background…………………………………………………7 1.0 Heat treatment of AISI 8620 steel…………………………………..8 2.0 Rust formation……………………………………………………13 3.0 Cleaning method……………………………………………….....15 Chapter 3 Journal Manuscript.. The effect of rust on gas carburizing of AISI 8620 steel………………….20 4 Chapter I Introduction 5 Introduction Surface contamination during heat treatment process can greatly affect the quality of the heat treated parts. Although cleaning the post heat treated parts is considered a value added process in heat treatment, cleaning pre heat treated parts is also important and can influence the subsequent process. The carburizing process can be affected by surface contamination, such as rust. The contaminant on the surface of the part may act as a diffusion barrier layer. AISI 8620 steel is the hardenable chromium, molybdenum, nickel based low alloy steel often used for carburizing to develop a case-hardened part. After carburizing, the steel provides, uniform case depth, hardness and wear properties, and gives the advantage of low distortion. Literature review has been done for the AISI 8620 steel carburizing, rust formation and cleaning methods which are used to remove rust. The objective of research is to study the effects of rust on the gas carburizing process and evaluate the efficiency of acid cleaning used to remove the rust. The effect of rust on the carburization behavior of AISI 8620 steel has been experimentally investigated. Hardness after carburizing is used as the parameter to evaluate the heat treatment performance. These results are also discussed in terms of the effects of carbon mass transfer on the steel surface and the resulting mass transfer coefficient. To determine the effect of rust on gas carburizing, weight gained by the parts and the surface hardness were measured. Surface carbon concentration was also measured using mass spectrometry. Carbon flux and mass transfer coefficient have been calculated. 6 Chapter II Background 7 1.0 Heat treatment of AISI 8620 steel Carburizing is one of the most widely used surface hardening processes. The process involves diffusing carbon into a low carbon steel alloy to form a high carbon steel surface. [1] Carbon transfers from gas atmosphere through the boundary layer, reacts with the steel surface in vapor-solid interface and then diffuses into the bulk of the material. During diffusion, there are several controllable parameters which can be adjusted to meet the customer’s tolerances and specifications, including carbon potential atmosphere, temperature and time. The maximum carburization rate can be achieved by controlling the rate of carbon transfer from the atmosphere and the rate of carbon diffusion into the steel. Carburizing process performance strongly depends on the process parameters, as well as furnace types, materials characteristics, atmosphere etc. All of these factors contribute to the mass transfer coefficient (β) which relates the mass transfer rate, mass transfer area, and carbon concentration gradient as driving force. So the mass transfer coefficient and the coefficient of carbon diffusion in steel are the parameters that control the process. [2-4] The total quantity of the carbon which diffused through the surface can be estimated by integrating the concentration profile over the depth of the carburized layer. Furthermore, differentiation of the total weight gain by the carburizing time yields the following expression for the total flux of carbon atoms through the vapor/solid interface. [5] The flux of carbon atoms diffused in the workpiece through the interface can be presented as shown in equation (1): ∂Δ⎛⎞M J = ⎜⎟ (1) ∂tA⎝⎠ where J is the carbon flux (g/cm2*s), ΔM is the total weight gain (g), A is the surface area (cm2) and t is the carburizing time(s). 8 The flux in the atmosphere boundary layer is proportional to the difference between the surface carbon concentration in the steel and the atmosphere carburizing potential, the mass transfer coefficient can be presented as follows [6]： ∂ x0 ∫ Cxtdx(), ∂t x ()ΔM A β ==∞ (2) ()CCP −−SPS tCC() where β is the mass transfer coefficient (cm/s), Cs is the surface carbon concentration in the steel, and CP is the atmosphere carburizing potential. AISI 8620 steel is a hardenable chromium, molybdenum, nickel based low alloy steel often used for carburizing to develop a case-hardened part. The well balanced alloy content permits hardening to produce a hard wear resistant case combined with core strength on the order of 862 mPa (125,000 psi). It has excellent machinability and responds well to polishing applications. With the balanced analysis, this steel provides, uniform case depth, hardness and wear properties, and gives the advantage of low distortion. [7] The standard carburization for AISI 8620 is at 900 oC to 925 oC o (1650 to 1700 F) in an appropriate carburizing medium (Cp = 0.8-1.2 wt% C) and quenched in oil to enhance the surface hardness. Improved carburized case and core properties can be obtained by furnace cooling from carburizing at 900 oC to 925 oC (1650 - 1700oF) and then reheating to 860oC (1575oF). Carburizing is accomplished at the same range of 900 oC to 925 oC (1650 to 1700oF) in a carburizing environment, followed by oil quench. [8] Fig.1 depicts an schematic illustration showing the locations on the Fe-C phase diagram of the conventional heat treatment in the core of the surface-carburized AISI 8620 steel.[9] During the heat treatment at 900 °C (points a and b), both the carburized surface (0.8% C) and the core of the specimen (0.2% C) remained in the austenitic single-phase region. Oil quenching from 900 °C to room temperature produced a microstructure that nearly all martensite throughout both the core and the case. 9 Figure 1. Schematic illustration of part of the Fe-C phase diagram showing the locations of the heat treatment in the core of surface-carburized AISI 8620 steel. [9] Izciler, and Tabur [10] examined abrasive wear behavior of different case depth gas carburized AISI 8620 gear steel. In their research, 320 min and 660 min at 925 °C in an endothermic atmosphere with constant 0.16% CO2 presence carburization condition were used. Homogenous matrix at the cross section and composed of pearlite and ferrite microstructure were seen in the core, as shown in Figure 2. Hardness measurements of the specimens were done before and after the heat treatment on a straight line from core to the surface by intervals of 1 mm under the load of 5 kg, the results are shown in Figure 3 and 4 respectively. 10 Figure. 2. Microstructure of AISI 8620 steel (core).[10] Figure 3. Hardness distrubution of the specimens.[10] 11 Figure 4 Carburized case depth of 320min specimen (from surface to end of case, total case depth 0.86mm).[10] Erdogan, and Tekeli [9] also investigated carburized AISI 8620 steel. The cross section hardness profile is shown in Figure 5. The carburized surface and the core of the specimen remained in the austenitic single-phase region.