Quenching distortion in AISI E52100 steel Hans Kellner Master of Science Thesis MH210X Materials Science and Engineering, KTH 1 2 Abstract Heat treatment of different steel products have existed for thousands of years. It has always been an important tool to get the microstructure and resulting properties such as hardness and case hardness and it is even more important today than ever before. This project concentrated on the quenching process and means to decrease the distortion caused by this process. The effect of different oils, temperatures, agitation and if gas quenching could give better results were investigated. The results showed that Miller´s 75 quench oil was better than Park´s 420 at slow agitation and that the viscosity of the oils influenced how much changes in agitation speed and oil temperature affected the distortion. It also shows that gas quenching is an alternative to oil quenching if the microstructure can be improved. Otherwise using Miller´s 75 with low agitation in the Surface combustion furnace will give best results. 3 4 Sammanfattning Värmebehandling av olika järn produkter har existerat i tusentals år. Det har alltid varit ett viktigt redskap för att få dem mikrostruktur och resulterande egenskaper så som hårdhet och det är ännu viktigare idag än tidigare. Detta projekt koncentrerade på härdningsprocessen och möjligheterna att minska deformationen orsakad av denna process. Effekten av olika oljor, temperaturer, omrörning och om gas är ett alternativ var undersökt. Resultatet visar att Miller´s75 härdnings olja var bättre än Park´s 420 vid långsam omrörning och att viskositeten av oljorna påverkar hur mycket förändringar i temperaturen och omrörningen ändrar deformationen. Det visar också att gaser är ett alternative till olja vid härdningen om mikrostrukturen kan förbättras. Annars så gav Miller´s 75 olja med långsam omrörning i surface combustion´s ugn det bästa resultatet. 5 6 Abbreviations HPGQ High Pressure Gas Quenching LOM Light Optical Microscope Troostite Old name for fine perlite USL Upper Specified Limit LSL Lower Specified Limit SL Specified Limit UCL Upper Control Limit LCL Lower Control Limit ALD The company “ALD Thermal Treatment” Ms Temperature when martensitic transformation starts M50 Temperature when martensitic transformation has reached 50% AISI The steel standard specified by American Iron and Steel Institute BCC Body Centered Cubic BCT Body Centered Tetragonal FCC Face Centered Cubic 7 8 Table of Contents Abstract ......................................................................................................................................................... 3 Sammanfattning ............................................................................................................................................ 5 Abbreviations ................................................................................................................................................ 7 Introduction ................................................................................................................................................ 11 Heat treating processes .......................................................................................................................... 11 Distortion ................................................................................................................................................ 11 Quenching process .................................................................................................................................. 12 Oil Quenching .......................................................................................................................................... 15 Gas Quenching ........................................................................................................................................ 17 AISI E52100 ............................................................................................................................................. 18 Objective ..................................................................................................................................................... 19 Experiments ................................................................................................................................................ 20 Results and discussion ................................................................................................................................ 23 Conclusions ................................................................................................................................................. 33 Future work ................................................................................................................................................. 34 Acknowledgements ..................................................................................................................................... 35 Bibliography ................................................................................................................................................ 36 Appendix ..................................................................................................................................................... 38 9 10 Introduction Heat treating processes There are many different processes that fall under heat treating. Two processes where the material is heated to a given temperature and then cooled down is normalizing and tempering. For the normalizing process the metal is heated to approximately 100°F (55 °C) above the austenitizing temperature to get a uniform size of the grains and uniform composition. It is then cooled down at an appropriate speed. Then there is tempering where the temperature is held below the austenitizing temperature and that is aimed to increase the ductility and toughness. Both of these methods theoretically fall under annealing whose broader definition only says that the material is heated up to and held at a suitable temperature and then cooled down at a suitable speed. Both the normalizing process and the tempering process can be used to relieve stress in the material but tempering is the most used one. While the tempering can cause precipitations the normalizing will give austenite again and you would have to quench it once more (1). Heat treating processes also include different hardening processes such as quenching, carburizing and carbonitriding. Quenching can be used to get both surface hardness and through hardness depending on how fast the material is cooled down throughout the part. Faster cooling give martensite while slower cooling can give ferrite, bainite and perlite. But usually when quenching is used you want to get martensite. Carburizing and carbonitriding on the other hand can only be used to get a hard surface layer and is done by heating up the material in a carbon rich atmosphere or with a powder on the material (1). When steel is quenched to get martensite you will get retained austenite in almost all cases. To transform this to martensite the material can be cold or cryogenic treated where the steel is cooled down to around -120°F (-84°C) or -310°F (-190°C) (1). These processes are put together in different ways and with different parameters to get the wanted effect on the material. Most of the time this means to get as little distortion as possible and the right microstructure. Distortion When a material is quenched there are three fields that will change. These are the thermal field, the metallurgical field and the mechanical field that distortion falls under (2). The distortion occurring during quenching can be divided into two main parts, shape distortion and size distortion. Shape distortion is when the product twist, bend or warp out of shape while size distortion keeps the form but the measurements change due to changing volume (3) (4). There are three main reasons for distortion during quenching. First there are microstructure changes that will induce transformation kinetics and transformation plastics. This will give slightly different 11 volumes when the phases change resulting in internal stress that when it exceeds the yield strength causes distortion of the part. Work on simulating this has been done by e.g. Seok-Jae et al. (5) using the FEM software ABAQUS. Their simulation of distortion in AISI 5120 during quenching was relatively close to the actual result from quenching. The second reason is gradients in the materials thermal field that will cause different expansion coefficients with stress as a result. Last of the three main reasons is residual stresses that can distort the material when the temperature is raised and the yield strength decrease. The residual stresses are mainly due to the machining of the parts and will vary somewhat during the lifetime of the tools used to produce the parts (3). Any distortion occurring will originate from one or more of these reasons. In Figure 1 the relationships between the three reasons described above and several others are shown. Figure 1. The relationship between the different fields and what affects them during the quenching process (2). In the study by Ashok et al. (2) it was concluded that the material properties that affect the residual stresses and distortion are the thermal conductivity, the Ms temperature and the shear modulus. Surm et al. (6) studied different causes for distortion and roundness deformation on bearing rings made of AISI E52100 and concluded that