Automotive bearing materials and performance requirements2018-08-28 12:41:06

At present, the car is developing in the direction of light weight, comfort, solidity, high power, compact structure, durability, high acceleration performance and high reliability. In addition, due to the advancement of electronic control technology, automobiles are also developing in the direction of practical performance such as low energy consumption, simple and convenient, and strong operability. Therefore, rolling bearings, as an important component for automobile support, must adapt to this trend.

During the service of the automobile bearing, the surface area of the rolling element and the ferrule is subjected to a large pressure. The calculated maximum is 5000 MPa. When the bearing is running, there is rolling and sliding, except for high frequency and alternating contact. In addition to stress, it is also affected by centrifugal force, as shown in Figure 6. According to the literature, the main failure modes of automotive bearings are spalling, pitting, adhesion, strain, fracture, loss of precision, and excessive vibration and noise [10-11]. Therefore, there are several requirements for the performance of bearing steel: (1) High purity. (2) Low oxygen content. (3) High hardness and wear resistance. (4) Good dimensional stability. (5) Sufficient compressive strength and resistance to deformation. (6) Good process performance.

Bearing steel is one of the important special steel varieties. Its quality and performance are largely reflected in the metallurgical level of a country. Automobile bearing steel accounts for about 40% of the annual output of bearing steel. The life and reliability of automotive bearings are of course related to the original design, manufacturing, lubrication, installation and maintenance, but raw materials are the key.

It can be seen from the above that in the major components of the automotive bearing assembly, in addition to the cage from the original stamping steel to the popular nylon material (such as injection molded PA66 + 30% GF) and seals are more commonly used rubber materials ( Such as ACM+SPCC, NB R+SU S430), the rolling element and inner and outer ring of China's automotive bearings are used in such a large amount of wC=0.95%~1.05%, w C r=1.4 0%~1.6 5%. Carbon-containing chromium bearing steel, in order to improve the hardenability to meet the changes of the wall thickness of the part, increase the M o content to develop a series of high-hardenability high carbon chromium bearing steel: for example, Germany's 100Cr6, 100Cr Mo, Sweden's SKF2 , SKF3, US 52100.3, 52100.4, Japan's SU J2, SU J3, SU J4, SU J5, etc., suitable for martensite quenching, also suitable for bainite quenching of ultra-thick wall bearing parts [12], There are subtle differences in chemical composition, and the schedule is a comparison of the two. In fact, these steels can be regarded as variants of GCr15.

Although its variety is relatively simple, it is also the most demanding of the structural steel (gear steel, bearing steel, spring steel, non-tempered steel, cold heading steel). The most demanding is the need to improve the purity of steel, strictly reduce the content of trace elements such as O, S, Ca, N, Ti, etc., but also control the defects that may be caused in metallurgical materials including smelting, pouring, rolling, forging [13] ], for example, smelting processes such as vacuum degassing, electroslag remelting, and furnace refining are often used. Reducing the oxygen content can significantly prolong the fatigue life of the bearing. Figure 7 shows the relationship between the oxygen content and the relative life of the bearing [14]. In the national standard GB/T 18254-2002, the O content of high carbon chromium bearing steel is clearly defined: die cast steel ≤ 15 × 10-6, continuous casting steel ≤ 12 × 10 -6 [15], in actual production With the development and control of metallurgical equipment and processes more stringent, the level can reach 5 × 10-6. In addition, standard or technical agreements have limitations on smelting methods, non-metallic inclusions, segregation, decarburization, low-magnification, microstructure, carbide inhomogeneity, surface quality, dimensional tolerances, etc. And the bearing production enterprises must strictly test and strictly manage the steel when it enters the factory, and it is not sloppy.

It should be added that, except for the majority of the automotive bearings, only the crankshaft bushings, the connecting rods, the piston bushings, the damper guides, the transmission reverse gear bushings, etc. are sliding bearings, which do not involve heat treatment. Mainly relying on powder metallurgy sintering + rolling method, the material is generally made of steel with wC≤0.15% on the back (such as 08Al), and the main bearing is alloy layer (such as Al-Sn20Cu or common tin base, lead base, copper). Base or aluminum based bearing alloy). The heat treatment discussed later is also directed to automotive rolling bearings.

GCr15 and 100Cr6 chemical composition (mass fraction) comparison table Download original table

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GCr15 and 100Cr6 chemical composition (mass fraction) comparison table
Figure 6 Loads when the bearing is running
Figure 6 Loads when the bearing is running Download the original image

Figure 7 Relationship between oxygen content and bearing life
Figure 7 Relationship between oxygen content and bearing life Download the original image

3. Automotive bearing heat treatment technology
The main role of the main alloying element chromium in the steel for automotive bearings is to improve the hardenability of the steel, so that the parts obtain a more uniform structure in the entire section after quenching and tempering. Chromium can form alloy cementite (Fe, Cr) 3C, which makes the austenite grain refinement, reduces the superheat sensitivity of steel when heated, improves the wear resistance, and can make the steel get fine needle shape or hidden when quenching. The crystal martensite increases the toughness of the steel on a high strength [16]. It is generally not necessary to cryogenically treat the automotive bearing unless there is a special requirement for the dimensional stability of the part and the retained austenite content.

(1) Production process route of automotive bearing parts The general production route of steel ball rolling elements is: bar material → ball billet hot forming → light ball (boring) → soft grinding → heat treatment → hard grinding → fine grinding → fine grinding (polishing ).

The general production route of the inner jacket ring is: pipe material (cold enthalpy) → annealing → turning → soft grinding → heat treatment → grinding → finishing.

The general production route of the cage is: solid cage (nylon): blank → turning → pulling, drilling window → surface treatment.

Stamping cage (metal): strip or sheet → forming → cutting bottom → punching window → pressing slope → expansion → surface treatment.

Rolling bodies, outer rings, inner rings of automobile bearings, and two or three generations of flanges of wheel bearings are properly heat treated to give full play to the potential of the material itself.