The depth of hardness on bushing wear resistance
As an index of material resistance to local plastic deformation, hardness has a strong correlation with bushing wear resistance, but the relationship between the two is not simple linear, but by the abrasion mechanism, working conditions, material microstructure and other factors. The following analysis from the role of principle, influence law, boundary conditions:
First, the basic principle of hardness and wear resistance
1. Inhibitory effect of hardness on wear mechanism
(1) Abrasive wear control
Abrasive wear control: When hard particles (such as iron filings and dust) are embedded in the surface of the bushing, the high hardness material (e.g. HRC60+ bearing steel) resists the particle indentation, so that the abrasive particles only produce surface furrows rather than deep cuts.
Data support: 45 # steel (HB200) in the lubrication environment containing SiO₂ particles (hardness HV1000), the wear rate of 0.5mg / h; and GCr15 (HRC62) wear rate down to 0.05mg / h, a drop of 90%.
(2) Adhesive wear inhibition
Microscopic process: low hardness materials (such as aluminum alloy) in high-speed friction, the surface metal is easy to soften due to local temperature rise and the occurrence of "cold welding", the formation of adhesive nodules; high hardness materials (such as quenched steel) surface peaks of high strength, the nodules are not easy to tear, reduce material migration.
Typical case: Unhardened 20# steel bushing (HB140) after 100 hours of operation without lubrication, obvious adhesion pits appeared on the surface; after carburizing and quenching to HRC58, the depth of the pits decreased from 0.15mm to 0.02mm.
2. Synergistic effect of hardness and surface microstructure
Strengthening by grain refinement: high hardness materials are often accompanied by fine grain organization (e.g. martensite grain size < 5μm in bearing steel after quenching), and the hindering effect of grain boundaries on dislocation motion is enhanced, which makes it more difficult to produce wear grooves on the surface.
The role of the second phase particles: Cemented carbide bushings (such as WC-Co) in the diffuse distribution of WC particles (hardness HV2000) as a "microscopic armor", when the matrix hardness (HRC65) and the second phase to match, the wear can be reduced by 70% compared to a single metal.
Second. Quantitative hardness - wear resistance relationship and critical thresholds
1. Linear correlation interval (dry friction condition at room temperature)
| Material Hardness (HRC) |
|
Typical application scenarios | ||
| 20-30 | 15-20 | Low-speed light-duty agricultural machinery bushings | ||
| 40-50 | 5-8 | Intermediate axle sleeve for automobile transmission | ||
| 60-65 | 0.5-1 | Precision machine tool spindle bushing |
Note: Data based on dry friction sliding wear test (load 50N, speed 0.5m/s)
2. Non-linear transition phenomenon (after exceeding critical hardness)
Risk of brittle wear: When the hardness exceeds HRC68 (e.g. ceramic bushings), the fracture toughness of the material (KIC<5MPa/m¹/²) decreases considerably, microcracking occurs under impact loading and the wear rate rises. For example:
Si3N4 ceramic bushings (HRC75) have a wear rate 3 times higher than HRC62 bearing steel under unlubricated impact conditions.
Optimum hardness range: The optimum hardness for wear resistance of most metal-based bushings is between HRC55-62, where the Vickers hardness (HV) and the logarithm of the wear rate are linearly negatively correlated (R²>0.92).
Third, the modulation of working conditions on the hardness - wear resistance relationship
1. The influence of lubrication state
Boundary lubrication: in the oil film is incomplete (such as Stribeck curve of the mixed lubrication zone), high hardness (HRC60 +) bushings can puncture the oxidation layer in the oil film, to maintain the stability of the boundary film, the wear rate than the low hardness of the material is lower than 40%.
Full-film lubrication: When the oil film thickness (h>1μm) completely covers the surface roughness, the impact of hardness is weakened. For example, the difference in wear rate between a copper alloy bushing (HB120) and a bearing steel bushing (HRC62) under dynamic pressure lubrication is < 5%.
2. Temperature and speed coupling effect
High temperature softening: 45 # steel (HRC40) in 200 ℃ when the hardness drops to HB180, wear rate than room temperature increases 2.5 times; and heat-resistant steel (such as 1Cr13, HRC50) in 300 ℃ when the hardness retention rate > 90%.
High-speed thermal effect: when the linear velocity > 10m/s, high hardness materials (ceramics with low thermal conductivity) due to frictional heat buildup leading to surface annealing, wear rate may be reversed to exceed the medium-hardness metal (such as bronze).
Fourth, hardness optimization strategy in engineering applications
1. gradient hardness design
surface hardening process: the use of nitriding (hardness of the infiltration layer HV900-1200), laser quenching (surface HRC65-70) and other technologies, so that the surface of the bushing hard core tough. For example:
After the camshaft bushing of automobile engine is ion nitrided, the surface wear is reduced by 60% compared with that of the whole hardened part, and at the same time, the brittle fracture of the core is avoided.
2. Matching of hardness and other properties
Hardness - toughness balance: Construction machinery bushings (e.g. excavator bucket shaft bushings) need to be hardened to HRC45-50, where the impact toughness (≥25J/cm²) prevents chipping due to rock impact and extends the life of the bushings by 1.8 times compared to HRC60 bushings.
Hardness - corrosion resistance coordination: seawater pump bushing selection of 316L stainless steel (HRC28-32), although the hardness is lower than the bearing steel, but the passivation film corrosion resistance so that the comprehensive life (8000 hours) than the chrome-plated steel (HRC60, life of 5000 hours) longer.
Five, typical failure cases and hardness correlation analysis
1. abrasive wear failure (mining machinery)
failure phenomenon: a crusher spindle bushing (45 # steel, HB220) running 3 months after the inner diameter wear exceeds 0.3mm, far more than the permissible value of 0.1mm.
Hardness attribution: Ore dust (hardness HV800-1200) far exceeded the surface hardness of the bushing, and it was recommended to use GCr15 bearing steel with HRC58-62, which was expected to extend the service life to more than 1 year. 2.
2. Adhesive wear failure (compressor)
Failure phenomenon: Aluminum alloy bushing (HB90) was clutched at start-up without lubrication, and metal transfer traces appeared on the surface.
Improvement measures: hard chrome plating (HV1000) on the surface increased the hardness by 10 times, and the critical speed of adhesion was increased from 2m/s to 8m/s, which successfully solved the problem of sticking.
Summary: The influence of hardness on the wear resistance of the bushing follows the law of "positive correlation within the effective interval, non-linear after exceeding the threshold value". In engineering design, it is necessary to combine the type of wear (abrasive/adhesion/fatigue), working condition parameters (load/speed/temperature) and material matching to control the hardness in the optimal range (usually HRC45-62), and to optimize the performance of "outer hardness and inner toughness" through surface strengthening technology, so as to maximize the wear life of the bushing.
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