ABSTRACT
The purpose of heat treatment is to refine the microstructure of the metal. Most carbon steel and carbon alloy steel can be heat treated for improving mechanical properties such as tensile & yield strength. 20MnCr5 is low alloyed engineering case hardening steel for parts which require core tensile strength of 1000-1300N/mm² and good wearing resistivity. It is used in box and piston bolts, spindles, camshafts, gear shafts and other mechanical controlling parts. Grain boundary oxidation is surface phenomenon that is most often associated with atmospheric gas carburizing. The rate of diffusion of oxygen into steel surface is dependent on the oxygen potential of the furnace atmosphere and the process variables (depth of oxide penetration is influenced by case depth, time at carburizing temperature, carbon potential and chemical composition of the steel.)
Keywords: Heat Treatment, Furnace, Lab Data Study Of GBO.
I. INTRODUCTION
20MnCr5 is case-hardening steel with low carbon content but good hardenability reaching good wear resistance due to high surface hardness after hardening. The small grain size benefits in good ductility and fatigue strength. Suitable for gearboxes and axle gears. During the carburization process, the oxygen atoms (which are about 35% smaller than the iron atoms) are released as a direct result of the presence of water vapor and carbon dioxide in the furnace atmosphere. Oxygen diffuses slowly into the steel surface (as does carbon and hydrogen, albeit more quickly) and migrates to the grain boundaries. Once in the steel, oxygen combines chemically with the elements already present (e.g. chromium, titanium, manganese) that have an affinity for oxygen.
The depth of the GBO layer can vary. For some neutral hardening in an Endothermic gas atmosphere, an GBO layer of 0.0075 mm (0.0003”) to 0.0127 mm (0.0005”) can be expected. For carburizing, an GBO layer of 0.025 mm (0.001”) or greater can be expected for case depths under 2 mm (0.078”). Heavier case depths will produce deeper GBO layers. GBO appears microscopically in the form of grain boundary penetration from the surface inward.
Carburized Component Exhibiting GBO to a Depth of Approximately 0.020 mm (0.007”)
During the gas carburizing process it is observed in the Laboratory that gear components of one gas furnace have higher Grain boundary oxidation against other five furnace. As shown in the figure above we can see the dark line is the garin boundary after oxidation. Actually Gas Carburizing Process is a surface chemistry process, which improves the case depth hardness of a component by diffusing carbon into the surface layer to improve wear and fatigue resistance. During gas carburizing work pieces are pre-heated and then held for a period of time at an elevated temperature in the austenitic region of the specific alloy, typically the temperature between 820°C and 940°C. During the thermal cycle the components are subject to an enriched carbon atmosphere such that nascent species of carbon can
diffuse into the surface layers of the component. The rate of diffusion is dependent on the alloy and carbon potential of the atmosphere. Care must be taken to ensure that only sufficient carbon is available in the furnace atmosphere at any one time to satisfy the take up rate of the alloy to accept the carbon atoms. Depending on the final requirement for effective case depth, the whole cycle may take many hours. Once the heating and carbon diffusion part of the process are complete it is necessary to rapidly quench the components to a defined alloy recipe. The recipe will specify the quench method, the quenchant temperature and time.
The purpose of the quench process is to provide the required hardness of the component by completing a Martensitic phase change in the alloy.
II. BACKGROUND OF THE EQUIPMENT
In the heat treatment workshop there are two types of furnaces. The figure is given below. But we are using Indogas furnace for gas carburizing. This furnace is sealed gas furnace. There are six no. of indogas furnaces in heat treatment workshop. We are using LPG gas as fuel for Indogas furnaces. LPG gas is hydrocarbon gas with carbon content. Normally due to increase in carbon potential the grain boundary oxidation will increases.
III. ANALYSIS OF THE PROBLEM
Gas carburizing allows for accurate control of both the process temperature and carburizing atmosphere
(carbon potential). Carburizing is a time/temperature process; the carburizing atmosphere is introduced into the furnace and work pieces are pre-heated and then held for a period at 910°C and 920°C for the required time to ensure the correct depth of case. The carbon potential of the gas can be lowered to permit diffusion, avoiding excess carbon in the surface layer.
After carburizing, the work is either slow cooled for later quench hardening, or quenched directly into oil. Quench selection is made to achieve the optimum properties with acceptable levels of dimensional change. Hot oil quenching may be used for minimal distortion, but may be limited in application by the strength requirements for the product. Alternatively, bearing races may be press quenched to maintain their dimensional tolerances, minimizing the need for excessive post heat treatment grinding. In some cases, product is tempered, then cryogenically processed to convert retained austenite to martensite, and then retempered.
Grain boundary oxidation is a surface phenomenon that is most often associated with atmosphere gas carburizing. The consequence of GBO (and the concentration gradients that develop during oxide formation) is that the material adjacent to the oxides has modified transformational behavior. Instead of forming martensite on quenching, steels with this condition develop non-martensitic transformation products (e.g. bainite, pearlite), which adversely affect mechanical properties (e.g. hardness, residual stress, bending fatigue).
In this report I studied the grain boundary oxidation for 20MnCr5 steel, that is used in manufacturing of gears with High case depth (1-1.3mm) and low case depth (0.4- 0.47mm). I studied both high and Low case depth for my report. During my study, I considered the following points that affect the GBO in High and Low case depth.
During this process grain boundary oxidized due to presence of oxygen.
I studied the effect of different element on grain boundary oxidation and analysis is given below.
1. Effect of % C on Grain boundary oxidation
As per below graph grain boundary oxidation is decreasing with increasing %C for high and low case depth. In 20MnCr5, %C lies between 0.17 to 0.22 %. Within (0.17 -0.18) %C the GBO is 26 microns for high case depth and 24.4 micron for low case depth, but in (0.19-0.22) %C GBO is decreased and for High case depth it is 23 microns and low case depth both it is 20micron.
2. Effect of Grain size on Grain boundary oxidation
In study on grain size, GBO is increasing with increasing Grain size for both case depth. Grain size lies between 5-8 (ASTM no.) for 20MnCr5. Within grain size 5-6 (ASTM no.) the GBO is 23 microns for high case depth and 20 microns for low case depth while grain size 7-8 the GBO is higher side for high case depth (26 microns) and lower-case depth (24.4 microns).
3. Effect of oxygen (O2 ppm) on Grain boundary oxidation
Oxygen (O2 ppm) also affecting the GBO in both high and lower case depth, As graph is showing that increasing oxygen (ppm) also increasing GBO.
For higher case depth GBO is 23 microns and lower case depth GBO is 20 microns in 1-10 ppm oxygen. In 11-20 ppm oxygen GBO is increased 24 microns for high case depth and 24.4 microns for lower case depth.
4. Effect of Carbon potential (Cp) on GBO
Increasing carbon potential also increasing GBO. In below graph at Cp (0.89-0.99) GBO is low in July (19 microns) and August (23microns) month but at Cp (1.0-1.20), GBO is is high both July (25microns) and August (26 microns) month.
IV. CONCLUSION
From the various results obtained during the project work it can be concluded that the grain boundary oxidation is vary depending upon the chemical element gas ppm, carbon potential. This study identified the desired material characteristics with regard to grain boundary oxidation in gears. The objective was to optimize the alloy composition of case carburizing steels for obtaining a load bearing characteristic providing better service performance of gears.
This study gives the guide line to control the oxidation in grain boundary of 20MnCr5 during carburization process. These are given below.
1. Increasing % C, decreases the grain boundary oxidation for High and low case depth both.
2. Increasing Grain sizes, increases the grain boundary oxidation for both high and low case depth.
3. Increasing oxygen (O2ppm), increases grain boundary oxidation for both high and low case depths.
4. Increasing Carbon potential (Cp), increases grain boundary oxidation.
Note:
This study has been done only for sealed quench furnace, in sealed quench furnace, maximum acceptable limit for grain boundary oxidation is 20 microns. But this limit will decrease for vacuum furnace.
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