Bulletin No. 4. ISO 9001 Registered HEAT TREAT BULLETIN. Case Hardening Of Steel Components And Straightening

Transcription

1 Bulletin No. 4 ISO 9001 Registered HEAT TREAT BULLETIN Case Hardening Of Steel Components And Straightening

2 The topics of discussion for this bulletin are: 1. Case hardening of steel components, and 2. Straightening. CASE HARDENING By definition, case hardening is a group of heat treatment procedures that develop a thin, hard surface layer on a softer, tougher core. The key words here are softer and tougher. The ideal type of steel suitable for case hardening is one that has less than.30% carbon or one that has little to moderate alloy content (also, with less than.30% carbon content). Almost any steel can be carburized if the carbon content of the atmosphere has a higher carbon content than the steel that is exposed to this atmosphere. For example, a 1020 steel will dissolve approximately 1.2% of carbon at 1700 F. The carbon diffuses into the steel and develops a high carbon surface layer. The carbon content (as well as the subsequent quenched hardness) decreases progressively from the surface, through the case, to the core. An example of a poor choice of a steel for carburizing is an AISI tool steel of the D-2 type. AISI D-2 steel contains: Carbon: 1.50%, Chromium: 12.00% The carbon content of the atmosphere (called the carbon potential) must be kept below 1.00% to avoid the embrittling condition of grain boundary carbides. The normal structure of a hardened D-2 steel contains massive carbides in a tempered martensitic matrix. These massive carbides are made from the combination of carbon and the various alloying elements present (iron, chromium, molyebdum, etc.). This reduces the carbon content available for the formation of the hard martensite, developed in quenching, to less than.80%. Therefore, D-2 will be carburized in a normal 1.00% carbon atmosphere. However, the resultant carburized and hardened case will be over-saturated, highly stressed and will probably crack during quenching. The core hardness of any carburized steel can be estimated by determining what would the hardness be if the same steel were quenched without being carburized. For example, an AISI 4150 steel, a one-inch cross-section area, oil quenched from 1500 F. in oil will develop RC 58/60 hardness with a 500 F. temper, both surface and core. A carburized plain carbon steel with a carbon content of % will develop a case hardness of RC 60-63, when water quenched, with a softer core hardness of RC However, water (brine) quenching is the most drastic of the three quenching medias (water, oil or air) and is conductive to excessive distortion and possible cracking in quenching. On the other hand, a low carbon, low alloy steel can be carburized and oil quenched to develop a case hardness of RC as quenched, and a core hardness of RC Oil quenching is much less drastic resulting in less distortion (movement) and much less probability of cracking during quenching. 2

3 Examples of suitable alloy steels for carburizing are: AISI 8620, 4615, 9310, 3312, etc. All of these alloys contain less than.30% carbon. Examples of steels that are not suitable for carburizing are: AISI 4140, 4150, 6150, 4340, 1045 thru 1060., Again, the purpose of carburizing any steel is to develop a hard, wear-resistant case and a softer but tougher core. The popular opinion today appears to be "the harder the steel - the better". The fallacy of this claim can be realized using AISI 4150 as an example. Conventional hardening (without carburizing) if AISI 4150 using a 2-inch cross-section area will develop a hardness of RC with a minimum temper of 400 F. (surface to core). The same steel carburized, quenched and tempered at 400 F., will develop a surface hardness of RC and a core hardness of RC The core at a hardness above RC 50 is not tougher and with the necessity of maintaining a high case hardness, the heat treater must restrict the tempering temperature so as to retain the high hardness. This lower tempering temperature results in a more brittle core structure. If a higher hardness is desired, an upgrading from AISI 4150 to AISI L-6 is indicated. AISI L-6 can develop a hardness of RC in section sizes as large as 5 inches in cross section - without carburizing. The specification of case depth is divided into three groups: 1. Shallow case: inch 2. Medium case: inch, and 3. Deep case:.060 to as much as.250 inches The principle method used to achieve a shallow case depth is carbonitriding. Carbonitriding is a case hardening process for carbon and alloy steels that consists of holding the steel above the upper critical temperature in an atmosphere containing both carbon and nitrogen. This can be done in an atmosphere-controlled furnace or in a molten salt, which contains the two elements - carbon and nitrogen. These two elements harden the case in two different ways: 1. The carbon provides the hard martensitic case when the steel is quenched, and, 2. The nitrogen also increases the hardness but it reduces the critical cooling rate that allows the quenching to be done in oil, thereby reducing distortion. The medium case depth ( inch) is recommended for general purpose applications, while the deep case depth ( inch) for applications requiring a deeper effective case (effective case is the depth of the case below the surface which measures a RC 50 reading). Any carburizing operation simply adds carbon to the surface. This means that the carburized steel must be quenched in order to develop any significant hardness in the case and in the core. The surface (case) is altered chemically by an increase of carbon content. 3

4 A steel with.20% carbon will have a surface carbon content of 1.00% if the carbon potential of the atmosphere is 1.00%. If the carbon potential is.80%, the surface will contain.80% carbon, etc. When a steel is selected for a component that will be surface hardened by carburizing and quenching several very important considerations must be made as to the design of the component: 1. If the steel has a carbon content above.30%; as previously discussed, the core will tend to be brittle. 2. If too much finish grind stock is allowed, the hard part of the case will be removed. Again, the case diminishes in both carbon content as well as in quenched hardness from the surface through the depth of the case to the core. 3. A very critical consideration in the design of any component to be carburized is whether any area of the component is thin enough that the carbon will penetrate the entire cross section. This will result in a very brittle condition with a high probability of that area developing a crack during quenching. In our bulletin #3, we discussed the ever increasing difficulties encountered in the use of components made from plate steel products. The direction and magnitude of movement during the volumetric shape changes that occur during quenching makes it extremely difficult to allow the proper amount of finish stock needed for finishing by grinding. As an example, let us presume that we have a ring made from a AISI 4140 burnout. Since oil hardening steels tend to expand on quenching, we will allow a finish allowance of.030 inch (.015 inch per side). The desired finish hardness - RC If the steel ring does expand within the.015 inch per side allowed, the final ground surface will have a hardness within the range of RC If the ring shrinks.010 to.015 inch, there will not be enough grind stock left to finish the part. 3. If the ring expands more than the allowed.015-inch per side, most of the case will be ground off in the finishing operation. Note: A carburized and hardened case.050-inch deep will maintain RC hardness to a depth of about.010 inch below the surface. Because the carbon content decreases, so will the quench hardness decrease through the case. Hardness vs. Wearability and toughness can be described into three hardness ranges: 1. A hardness above RC 55 is considered hard for maximum wearability. Above this hardness there is strength and wearability with the least ductility (toughness). 2. A hardness range of RC contributes to a combination of wearability, strength and a good degree of ductility. 3. A hardness range of RC results in the maximum toughness sacrificing both strength and wearability. The hardness range in this range permits good machinability without the necessity of further heat treatment. (Brake die steels fall in this category). If a steel such as AISI 4150 develops a quench and tempered hardness of RC without being carburized and a case hardness of RC 60 with a core hardness of RC 52-55, the carburized and hardened component will tend to be more brittle than, as explained above. 4

5 The increase in the wearability of a steel of this type, with a through hardness in the range of RC as compared to the same steel carburized with a surface hardness of RC 60 and a core hardness of RC will hardly be realized while the brittleness of the core in the carburized component may be a problem in the final grinding operation as well as in service. STRAIGHTENING When any steel is quenched for hardening, internal stress will cause the steel to distort by expanding, contracting, bowing or twisting. In our previous bulletin we discussed "inherent" distortion. The steel will either grow of shrink simply by the formation of martensite (the hard phase). The heat treater cannot predict or prevent this distortion from occurring. Some steels are referred to as "non-deforming" steels. No steel that undergoes a volumetric size change is free from distortion. The amount of distortion depends on the shape of the component, the quench media, and the analysis of the steel. The steels that have the least distortion are the air hardening steels. The water or brine quenching steels exhibit the greatest amount of distortion, while the oil hardening groups fall in-between. The shape of a component determines to a large extent, the direction and magnitude of the total distortion. Long, thin designs are the most critical and the longer and thinner the component is, the greater will be the warpage by twisting and or bowing, regardless of the type of steel involved. Straightening can be accomplished by: 1. Hydraulic pressure 2. Pressure and heat (torch heating) 3. Peening 4. Hot pressure straightening Torch heating often tempers the heated area thereby causing "soft spots" which may render the component unusable. The potential for the transverse cracking of long, thin components is there for any method of mechanical or thermal straightening. The greater the quenched distortion, the more heat or heat and pressure is required to straighten the component. Peening is a mechanical method of straightening without the use of heat or pressure. A cemented carbide wedge is braised onto a hammer, which is used to impinge the surface of the part in order to cause the internal stress to be diverted to another area thereby reducing the distorted area. Peening leaves the surface with impingement marks, which may not clean up in the finished operation. Hot press straightening is best performed on air hardening steels. Air hardening steels transform to the hard martensitic structure below three hundred degrees. By using pressure on the part as its structure is passing through the transition zone, it is easily moved and straightened most effectively. When too little final grind stock is allowed more than the normal amount of heat or pressure is needed to bring the components to the straightness specifications required. When this component is finish ground the stress of grinding can offset the straightening stress causing the part to go out of tolerance. 5

6 Excess straightening is not only dangerous; it is also costly. If more stock is allowed (to be removed in the final grinding operation), the less straightening would be required, which would result in less scrap and less over-all cost. Written and published by East-Lind Heat Treat, Inc. for exclusive use by its customers. All technical questions regarding this bulletin may be directed to Dale Greer, Quality Manager. To order additional copies of this or other bulletins please contact our office at (248) EAST-LIND HEAT TREAT, INC Dequindre Rd. Madison Heights, MI ATTN.: PLANT MANAGER IMPORTANT INFORMATION 6