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Bearing Materials - SHANDONG OBETTER BERAING CO., LTD
6.Dec. 2010 WWW.OBTBEARING.COM
Martensitic Stainless Steel – SV30
Martensitic stainless steel can be modified during the processing of the raw steel by lowering the carbon content and introducing nitrogen as an alloying element. The nitrogen increases the saturation of the chromium which transforms into chromium nitrides, instead of chromium carbides. The result is a high strength, high hardness steel with a superior microstructure that extends fatigue life by as much 100% (double) in certain applications. This material also offers better corrosion resistance than 440C and ACD34, up to 5 times. Bearings manufactured from this material can carry a price premium of 20-40%. More information is available in the SV30 Technical Information Sheet.
Heat Treatment of Bearing Steels
When bearing steels are in their soft (unhardened) state, metallurgists refer to their structure as being in the pearlite state. In order to harden the steel it must be heated to a very high temperature and then cooled very rapidly. When heated in the heat treat furnace to 17500 F, the structure transforms from pearlite to what is known as austenite. After quenching (very rapid cooling), the structure then transforms from austenite to martensite. Once transformed to martensite, the steel becomes very hard. However, at this point it is not considered "thermally stabilized". This is because not all of the austenite transforms into martensite during the quenching process. This phenomenon is called "retained austenite
If the steel is not thermally stabilized, the retained austenite will over an extended period of time (possibly years) transform into martensite. This transformation is accompanied by an increase in volume that is called metallurgical growth (not to be confused with thermal growth). Metallurgical Growth will cause a change in dimension and form of any steel parts such as bearings' even at room temperature.
While not a problem with low precision commodity type bearings, in high precision (ABEC 5P, 7P, 9P) miniature bearings this lack of dimensional stability can cause problems. In order to eliminate this unwanted metallurgical growth, the steel must be subjected to thermal stabilization. This is accomplished by repeated cycles of chilling at -1200 F and tempering to transform a large percentage of the retained austenite to martensite.
The tables below show the chemical composition of the stainless steel alloys discussed above.
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Designation
Country |
C% Carbon |
Si% Silicon |
Mn% Manganese |
P% Phosphorous |
Cr% Chromium |
Mo% Molybdenum |
Ni% Nickel |
S% Sulfur |
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AISI 440C
USA |
.95-1.2 |
1.0 max |
1.0 max |
.04 max |
16-18 |
.75 max |
.25 max |
.03 max |
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SUS440C
JAPAN |
.95-1.2 |
1.0 max |
1.0 max |
.04 max |
16-18 |
.75 max |
.25 max |
.03 max |
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9Cr18
CHINA |
.90-1.05 |
.75 max |
.75 max |
.035 max |
16-19 |
.75 max |
.23 max |
.03 max |
AISI316 Austenitic Stainless Steel
Bearing components made from 300 series stainless steel materials have greater corrosion resistance and are non-magnetic because of the low carbon content. However, the tradeoff is that this material cannot be hardened so the bearings can only operate under low loads and speeds. The surfaces of the bearing undergo a chemical reaction with the oxygen called a passivation process; the passive film developed on the surface protects the bearing from corrosion. The corrosion resistance is best when the bearing is not completely submerged in liquid (such as underwater applications). Bearings made from this material are usually special order items requiring minimum quantities; in addition they are more expensive.
Other 300 Series Stainless Steel
The bearing shields, seal washers and ball retainers are sometimes made from AISI303 or AISI304 stainless steel because they have moderate corrosion resistance and are better for forming into the various shapes.
Carbon Alloy Steel
Carbon steel materials are used to produce various components of bearings and have two basic types, Medium versus Low Carbon Alloy Steel.
Medium Carbon Alloy Steel
Bearings made from medium or low carbon alloy steel material are typically called “semi-precision” or “commercial grade” bearings. The typical materials are AISI8620 or AISI4320. The inner and outer rings are surface hardened in a heat-treating process call case hardening or carburizing. Bearings made from these materials cannot carry high loads or operate at high speeds and they do not have corrosion resistance. The bearings made from these materials are usually low cost.
Low Carbon Alloy Steel
Low carbon steel is used in the production of bearing cages, metal shields and metal washers around which rubber is molded for seals. Common materials are AISI C1008 and C1010. The material needs to be protected from corrosion with a coating of oil/grease (cages) or by plating (shields). Additional information on Retainers and Closures can be found in the respective technical bulletins.
Plastics and Non-Metallic Materials
Cages
Plastic materials are sometimes used for bearing cages. The most common is molded nylon plastic but molded acetal (POM) is also used. Other polymers are used in special bearing designs where specific performance requirements such as high speed, or low torque or low noise are identified.
Cages made from phenolic (phenol-formaldehyde) materials are most common in high speed ball bearings used in machine tool equipment spindles. The phenolic cages are lightweight and have high strength. The cost of processing this material has led to an increase use of plastics in its place. Additional information is contained in the Cages technical bulletin.
Seals
The most common seal material is nitrile rubber or Buna rubber. It has good mechanical properties, can be used across a wide temperature range, is resistant to many chemicals and is low in cost. Materials such as Viton and Silicone are expensive elastomers and they’re usually used when an application requires their unique properties.
Ceramic Materials
Bearings made with ceramic materials fall into a specialty niche in the bearing industry. The most common arrangement is a hybrid bearing, usually with stainless steel rings and ceramic balls. The most common ceramic material used is silicon nitride. Balls made from this material are hard, up to Rc78, and have a very smooth surface. Hybrid bearings are more expensive than all stainless steel bearing.
Shandong Obetter bearing Company is a leading global supplier in the areas of bearings, mechatronics, services. OBT Group as a high-tech enterprise and professional manufacturer specicalized in ball bearing and roller bearing. we can produce non-standard bearing and machine parts as our customers request. we always follows the principles of "Good Faith, Equality, Cooperation and Win-Win" to cooperate with our customers.
For additional information, please contact:
Company brand: OBT Bearing
Company Name: Shandong Obetter Bearing Co., Ltd
WebSite: www.obtbearing.com
Tel/FAX:86-635-8511972
E-mail: obtbearing@163.com
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