Wear Coefficients and Design Rules for Tool Steel
Wear is a system property, so a grade cannot be chosen from a hardness table alone. What can be quantified is how severe the wear situation is, which mechanism each motion and environment produces, which material classes are used against each of those mechanisms, and the limits that keep a contact in mild wear instead of severe wear. This page collects those four things: the dimensionless wear coefficient with its engineering limits, the wear situation matrix, the contact stress and hardness thresholds, and the design rule list.
One wear number that compares across mechanisms
The wear coefficient is the classic way to put very different wear situations on one scale. It is the volume of wear multiplied by the hardness of the wearing surface, divided by the applied load and the sliding distance, which makes it dimensionless. The higher the value, the more severe the wear situation, and because it is dimensionless the values for a steel on a bench test and a die in a press can at least be compared in order of magnitude.
| Wear mechanism | Sliding wear coefficient, (V × H) / (P × S) |
|---|---|
| Adhesive wear | 10-7 to 10-1 |
| Abrasive wear | 10-6 to 10-1 |
| Corrosive wear | 10-7 to 10-2 |
| Fatigue wear | ≤ 10-6 |
The observed range of the dimensionless sliding wear coefficient for each mechanism, from the source table, where V is the volume of wear, H the hardness of the wearing surface, P the applied load and S the sliding distance. The values are taken from tribosystems in which that mechanism is the predominant one.
Two engineering limits come with the table and both are useful when a customer asks what a wear rate should be. Most engineering applications require a wear coefficient of 10-5 or less. Automotive engine cylinder applications often require 10-8. A design that lands in the 10-3 or 10-2 range is not in mild wear, whatever the grade.
The same table shows why the two mechanisms a tool steel is bought for are the hard ones to solve. Adhesive and abrasive wear span the widest ranges and sit at the severe end, and both are controlled by the lubricant and the contact as much as by the steel. Corrosive wear and fatigue wear sit at the mild end, which is why a tool that fails by either of them is usually failing for a reason other than hardness.
Which mechanism each motion and environment produces
The second table is an operational classification rather than a materials table. It answers the question that comes before grade selection: given this motion and this environment, which mechanisms can appear, and what does the source say about how the situation behaves as the operating conditions change. The practical use is that it tells you which of the four mechanisms to design against before any grade is named.
| Motion and environment | Mechanisms involved | What the source says |
|---|---|---|
| Rolling, two body, no slip | Fatigue | The mildest wear situation. Smooth surfaces are preferred and, with pure rolling, fatigue is the predominant mode. |
| Rolling with slip, and rolling with particles | Fatigue, adhesive, abrasive | Wear increases with increasing slip and with the presence of particles. Adhesive and abrasive modes can predominate once slip or particles are present. |
| Impact against a stationary body | Fatigue (elastic or plastic), adhesive | Induced vibration and misalignment can cause fretting, which raises wear. Plastic deformation is generally unacceptable except in short life applications, and for lives above 106 cycles the contact stresses need to stay in the elastic range. |
| Impact against a moving body | Fatigue (elastic or plastic), adhesive, abrasive | Wear increases with the amount of sliding and sliding effects can predominate. Fluid lubrication effects can be very significant, and the presence of particles raises wear. |
| Sliding, unidirectional, dry, fluid or with particles | Fatigue, adhesive, abrasive | More than one mechanism is involved. Fatigue is the mildest and the conditions that minimise adhesion and abrasion are preferred, with low contact stresses. With particles present, abrasion tends to become the predominant mode. |
| Sliding, cyclic, small amplitude | Fatigue, adhesive, abrasive | Mild to severe wear transitions are often associated with the transition from elastic to plastic deformation, and the predominant mechanism can change as wear proceeds. In mild wear situations the terminal mode is often fatigue. |
| Impingement, low angle, with particles | Abrasive (cutting) | The particle cuts the surface rather than deforming it, which is why low angle impingement is the abrasive form of erosion. |
| Impingement, high angle, with particles | Fatigue and abrasive (deformation) | At high angle the particle deforms the surface, and the damage is a mixture of deformation and fatigue rather than clean cutting. |
| Flow, streamline, with particles | Abrasive (cutting) | Without particles the mechanism in a flowing fluid is cavitation, and impinging fluid droplets act like particles. Corrosion effects are often present as well. |
| Flow, turbulent | Fatigue, abrasive | Turbulence raises the damage over a streamline flow, and with particles the abrasive component appears alongside the fatigue one. |
Operational classification of wear situations, condensed from the source table. The source adds a general note that with fluids other than lubricants, synergistic effects between corrosion and wear often occur, that oxidative wear is probable with metals and ceramics in all situations, and that the operational characterisation may differ at different locations on the same part.
Which material class is used against each wear situation
The matrix below is the one table in this set that says as much about what a tool steel is not for as about what it is for. A cross means that class is used in that application in practice. Tool steels carry a cross in nine of the twelve columns, which includes all three sliding and all three rolling situations, impact wear and three-body abrasion, and cavitation. They carry no cross in fluid erosion, drop erosion or particle erosion, where the table turns to carbides, ceramics and elastomers instead. That is the honest boundary: a tool steel is the answer to sliding, rolling, impact and abrasion, and it is not the answer to a stream of hard particles or drops.
| Material class | Sliding, unlubricated | Sliding, lubricated | Sliding, abrasion | Rolling, unlubricated | Rolling, lubricated | Rolling, abrasion | Impact | Three-body abrasion | Fluid erosion | Cavitation | Drop erosion | Particle erosion |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Structural alloys | ||||||||||||
| Surface treatments | X | X | X | X | X | |||||||
| Hard surfacing | X | X | X | X | X | X | X | X | ||||
| Soft coatings | X | X | X | |||||||||
| Alloy steels | X | X | X | X | X | X | X | X | ||||
| Tool steels | X | X | X | X | X | X | X | X | X | |||
| Stainless steels | ||||||||||||
| Precipitation hardened stainless | ||||||||||||
| Martensitic stainless | ||||||||||||
| Cast irons | ||||||||||||
| Graphitic cast iron | ||||||||||||
| White cast iron | ||||||||||||
| High-temperature alloys | ||||||||||||
| Refractory materials | X | X | X | X | ||||||||
| Superalloys | X | X | X | X | X | |||||||
| Copper-base alloys | ||||||||||||
| Bronze | X | X | ||||||||||
| Beryllium copper | X | X | ||||||||||
| Soft bearing alloys (babbitts) | X | |||||||||||
| Carbides | X | X | X | X | X | X | X | |||||
| Ceramics | X | X | X | X | X | X | ||||||
| Polymers | ||||||||||||
| Thermosets | X | |||||||||||
| Thermoplastics | X | X | X | |||||||||
| Elastomers | X | X | X | X | X | X | X | |||||
| Carbons | X | |||||||||||
| Lubricating composites | X | X |
Typical wear applications for selected engineering materials, reproduced from the source table, in which a cross marks the wear situations each material class is used in. Surface treatments, hard surfacing and soft coatings are grouped under structural alloys in the source, and the stainless steels, cast irons, high temperature alloys, copper base alloys and polymers are each split into the families listed here.
The limits that keep a contact in mild wear
Three thresholds decide whether a design wears mildly or severely. In a two body non-abrasive situation the contact stresses should stay in the elastic range, and for sliding they should be a small fraction of the yield strength: generally under 0.5 of yield, and under 0.2 when a very low wear rate is required. For rolling and impact the stresses can be significantly higher, above 0.5 of yield, but only while sliding is not involved.
Where hard particles are present the rule changes shape. For abrasive or deformation wear to occur at all, the wearing surface has to be softer than the particle or the asperity causing the damage, so a surface harder than the abrasive removes the cutting mechanism. It does not remove wear: the source is explicit that fatigue and fatigue-like processes remain possible and are generally milder. That is the difference between designing out abrasion and designing for a longer life.
Two cautions sit above all of this. Mild to severe wear transitions can be sharp and are often triggered by something outside the design, such as a change in humidity or in the particles arriving at the contact. And material rankings change with the test: resistances measured in an abrasive test are usually not the same order as those measured without abrasion, lubricated rankings differ from dry ones, and high speed rankings differ from low speed ones. A wear test that is meant to rank materials has to reproduce the loading, the contact geometry, the motions and the environment of the application, which is why a bench number never closes a grade decision on its own.
One technique the source recommends when the numbers are thin is bracketing: build two theoretical projections of the same design, one optimistic and one pessimistic, using different wear models, and treat the two as the bounds of the expected life. For sliding, the usual pair is the K factor model against the combination of the zero wear and measurable wear models.
Design rules for wear applications
The list below is the source rule set, in its own words and its own order. The rules that come up most often on tool steel work are the ones on dissimilar materials, on lubrication, on keeping the contact in mild wear, and on galling, which is why the galling threshold stress and the wear mechanism control for each mechanism are given on the pages linked below.
| Design rule for wear applications | Design rule for wear applications |
|---|---|
| 1. Reliance on analytical design procedures increases the degree of conservatism that should be used. | 14. The thickness of conventional coatings should generally be greater than 100 µm. |
| 2. Wear is a system property, so utilise all the parameters that influence wear. | 15. Use moderate surface roughness. |
| 3. Design with the limits and the characteristics of the materials in mind. | 16. Avoid the use of stainless steel shafts with impregnated sintered bronze. |
| 4. Design so that a mild wear condition exists. | 17. When moulded, filled plastics tend to show a significant difference between initial and long term wear behaviour. |
| 5. Minimise exposure to abrasive particles. | 18. When glass or other hard fillers are used, the hardness of the counterface should be equal to or greater than that of the filler. For glass this hardness should be above 60 HRC. |
| 6. Optimise the contact to minimise stresses, ensure good alignment, and round corners and edges. | 19. The tendency for galling can be reduced by using dissimilar and hard materials of low ductility, lubricating, and reducing contact stresses. Stress levels above the threshold for galling should be avoided. |
| 7. Use a lubricant whenever possible. | 20. Avoid designs in which fretting motions can occur. |
| 8. Use dissimilar materials. | 21. When fretting motions are present, design for optimum sliding wear life and to minimise abrasive wear. |
| 9. To increase system life it is sometimes necessary to increase the hardness of both members. | 22. Sacrificial wear design should be considered when satisfactory life cannot be achieved by other means. |
| 10. Rolling is preferred over sliding. | 23. Conform to vendor recommendations for optimum wear performance. |
| 11. Sliding or fretting motions should be eliminated in impact wear situations. | 24. Changes associated with design modifications or new applications should be reviewed carefully for their effect on potential wear behaviour. |
| 12. Impacts should be avoided in sliding contacts. | 25. Wear is a system property, so a material that wins on one bench test may not win on the job. |
| 13. Elastomers frequently outperform harder materials in impact situations. |
Design rules for wear applications, reproduced from the source list. The final entry restates the system property point that the source makes before the list: a material that wins on one bench test may not win on the job.
Where these rules meet a specific grade
The mechanisms themselves, and the metallurgical factor that moves each one, are on the tool steel wear mechanisms page. Where the failure has already happened and the question is which mechanism caused it, the reading order is on the damage mode identification chart. For pickup rather than wear on stainless work, the measured galling thresholds are on the galling of stainless tool steels page, and the surface treatments that harden the working face are compared on surface hardening of tool steel and the case hardening process comparison.
Before you use these values
This page is a reference summary of published practice and it is not an Aobo Steel specification. The wear coefficient ranges, the application matrix and the rule list are reproduced from the source, and the source itself warns that the rankings of materials change with the wear application and with the test used to obtain them. The useful answer for a specific tool comes from the combination of grade, counterface, lubricant and contact stress on the job.
Source: ASM Handbook, Volume 20, Materials Selection and Design, ASM International, 1997, from the article on design for wear resistance (Table 1, Table 2, Table 5 and Table 7).