Wear Mechanisms in Tool Steels and What Controls Them
A tool steel wears by one of four mechanisms, and each one answers to a different set of properties of the steel. That is why changing grade on a die that is failing for the wrong reason changes nothing. This page sets out the four mechanisms, the metallurgical factors that are reported to move each of them, what the measured data do and do not support, and the interface friction figures of the metalworking processes in which tool steel does its work.
Four mechanisms, one of which is doing the damage
The published account of tool steel wear names abrasion, adhesion, corrosion and contact fatigue as the four basic mechanisms. In metalworking they arrive together, and the proportions between them are what decides the useful life of the tool. Adhesion is usually the life limiting mechanism in dry forming, and lubrication is so effective against it that once the lubricant is correct, abrasion becomes the limiting one instead. Because the mixture varies with the job, the published advice is to select on production experience with the actual combination of steel, lubricant and coating rather than on a property table alone.
| Mechanism | How it presents on the tool |
|---|---|
| Abrasion | A hard phase is rubbed against the tool steel. The hard phase can be embedded in the workpiece, as oxide inclusions are in steel, or it can arrive in a fluid, as grinding dust reaches a guide through the lubricant flow or as contaminant reaches a pump through the oil. |
| Adhesion | Metal is cold welded between tool and workpiece where direct contact happens, which is where the lubricant film has broken down, where the pressure is high or where the process leaves no room for a film. It becomes worse as the oxygen content of the environment falls, because no oxide layer can form. |
| Corrosion | Fresh tool steel is exposed by the sliding, the corrosion film grows on it and the workpiece then ruptures that film, uncovering new steel. It is rarely the leading cause of tool wear but it can become the dominant one at high temperature and in plastics processing. |
| Contact fatigue | Repeated sliding or rolling of one component over another spalls the surface and starts microcracks. On a tool this shows up as chipping along a cutting edge or a punch edge rather than as a wear scar. |
The four wear mechanisms of tool steels and how each one appears in service, summarised from the article on friction and wear of tool steels in ASM Handbook, Volume 18. The source notes that these mechanisms rarely act alone and that the balance between them is set by the process.
What moves abrasive wear
Abrasion is the mechanism with the clearest structure behind it. In the carbon tool steels the abrasion resistance is directly related to the hardness and to the carbon content, and a further measurement shows it rising linearly with the volume fraction of cementite, which is the same statement written in terms of structure rather than chemistry. Explicitly, the factors that raise the abrasion resistance of a water hardening tool steel are more carbon, higher surface hardness and a larger volume fraction of cementite, and the first of those drives the other two.
For the alloyed grades a controlled study of dry abrasive wear found a good correlation with a single structural parameter, the size of the primary carbides multiplied by their volume fraction multiplied by their hardness. The practical reading of that product is that the most abrasion resistant tool steel has a large volume fraction of large, hard carbides. Two cautions come with it. Abrasion is rarely the only mechanism running, since most wear in a tool room is metal against metal. And a large carbide can be torn out of the matrix and then act as an abrasive itself, which is a way for a highly alloyed grade to wear faster than its carbide content suggests. Overtempering produces the same effect by softening the matrix that holds the carbides.
What moves adhesive wear, and where the data disagree
The published list of metallurgical factors reported to reduce adhesive wear in tool steels includes higher hot hardness, the self lubricating behaviour of the graphitic grades, a small grain size, a large volume fraction of primary carbides and a small primary carbide size. Most of that list rests on empirical work, and the source is careful about it. A direct measurement of unlubricated tool steel against hardened 440C and of tool steel against annealed 1020 steel found no correlation between tool steel wear and carbide size, carbon content, carbide volume fraction, total alloy content or hardness, and no correlation with a combined structural parameter either.
The honest conclusion for a tool room is that abrasive resistance can be predicted from composition and structure and adhesive resistance cannot. Where the failure is adhesive, the answer is a trial on the actual pair, and the levers that cost least to try first are the lubricant and a surface treatment, since both act on the mechanism directly rather than on the bulk of the steel.
The limit case of adhesion has its own name. In high speed steel used for demanding cutting, the temperature at the tip and in the wear crater can approach 700 C, and at that temperature carbon and chromium diffuse out of the tool steel into the chip. Wear at that severity is called seizure or galling. The three remedies the source gives are to reduce the cutting speed, to coat the tool, and to improve the lubrication and cooling.
The friction at the tool and workpiece interface
Interface friction is the quantity that decides how much of the work goes into sliding rather than into deformation, and the published values show how wide the range is across the processes in which tool steel works. In hot extrusion the coefficient of friction runs from 0.015 to 0.2, which is the whole spread from a well lubricated glass film to an almost dry interface. In upsetting, which stands in for forging, the dry value is about 0.2 for aluminium and steel and about 0.1 for copper and brass, and lubrication pulls the whole set down into the 0.05 to 0.07 band. In cutting, the coefficient is reported between 0.25 and 0.55 and it depends on the workpiece and the fluid more than on the tool, with proper lubrication reducing friction by about 60 percent when cutting aluminium, about 84 percent when cutting copper and about 35 percent when cutting steel and stainless steel.
Temperature is what sits behind those numbers. The tool and workpiece interface in cutting can exceed 600 C, which softens the tool and accelerates wear, and the source records that correct cooling alone reduces tool wear by about one third. In forging most organic lubricants break down above roughly 200 C, aluminium stearate is reported useful to about 400 C, and much work is done above 1000 C where only graphite, glasses and metallic oxides such as copper oxide survive.
Lubricant selection therefore follows the process rather than the grade, and the published practice can be reduced to a short list. Light press forming of most metals needs little more than a light mineral oil or a soap. Severe forming of copper, iron and nickel base alloys needs extreme pressure mineral oils carrying sulphur or chlorine, or zinc stearate soap, and the most severe press forming of iron and nickel base work is done with a phosphate coated workpiece and soap. Titanium always needs at least an extreme pressure oil, moving to wax or soap as the forming gets harder. Aluminium extrusion is usually run without any lubricant at all, because a bridge, spider or porthole die must not trap lubricant in the pressure welded seam, and the protection against die pickup then comes from keeping oxygen away from the die. Copper oxide is a good lubricant and brass oxide is not, so brass and hard copper alloys are extruded with glass. Hot forging runs on graphite suspensions in water, grease or oil, with sawdust, chloride salts, talc and mica all in recorded use. Cutting uses mineral oils, water miscible oils, synthetic fluids, gases where a clear view of the cut is needed and solid lubricants for low speed work such as hand tapping and reaming.
Matching the remedy to the mechanism
| Mechanism | What actually changes the outcome |
|---|---|
| Abrasion | Hardness at the working temperature, carbon content, volume fraction of cementite, and the size, volume fraction and hardness of the primary carbides taken together. A powder metallurgy grade changes the size of the carbides, not their type. |
| Adhesion | Hot hardness, the built-in lubricity of the graphitic grades O6 and A10, small grain size, a large volume fraction of primary carbides and a small primary carbide size, all reported from empirical work. A coat or a change of lubricant is the practical lever. |
| Corrosion | Lubrication that keeps the fresh surface covered, a higher chromium tool steel, or a coating that does not react with the workpiece. |
| Contact fatigue | Higher toughness and higher hardness together, which pull against each other. Two ways to raise toughness without touching hardness are a powder metallurgy grade, which reduces carbide size, and a lower austenitizing temperature. |
The metallurgical factors that control each wear mechanism in tool steel, from ASM Handbook, Volume 18. Where two of the factors pull against each other, as hardness and toughness do for contact fatigue, the source notes that raising one must not be allowed to create a larger problem than the one being solved.
One further point belongs to the tool room rather than to the steel. Hardness and toughness act in opposite directions, so an edge that chips is not automatically cured by hardening it, and the two routes that raise toughness without lowering hardness are a powder metallurgy grade and a lower hardening temperature. The carbide side of the same argument is set out on the tool steel carbide types and hardness page, and the measured carbide volumes on the carbide volume fractions page. The grades that Aobo Steel keeps for wear limited work are grouped on the abrasive wear resistant tool steels page.
Grades and stock
Where the wear has already happened and the question is what the mechanism was, the surface pattern is the evidence and the reading order is on the damage mode identification chart. Where the tool lost a piece of its edge rather than wearing away, the pattern belongs to impact wear and spalling. Where two surfaces moved against each other through a very small amplitude, the mechanism is fretting and it is described separately.
On a die rather than a cutting edge the same four mechanisms are read through the process, and the process by process mapping is on the die wear and failure mechanisms page. Where the workpiece is stainless and the failure is pickup rather than wear, the measured thresholds are on the galling of stainless steels page.
Before you use these values
This page is a reference summary of published practice and it is not an Aobo Steel specification. The mechanisms and the friction values are reproduced from the source, and the source itself warns that the friction figures give an approximate magnitude only. The useful answer for a specific tool comes from the combination of grade, lubricant and coating on the job.
Source: ASM Handbook, Volume 18, Friction, Lubrication, and Wear Technology, ASM International, 1992.