Galling of Stainless Steels and How Much Stress It Takes

The threshold galling stress, grade by grade

In the button on block test a loaded button is rotated against a block of the same material until the surface tears, and the contact stress at which that happens is the threshold galling stress. A higher number is better. The entries marked as not galling ran the full range of the test without tearing, which is why they share one value, and the two unit columns are reproduced as the source prints them. Hardness is given as the source gives it, in either Rockwell B or Rockwell C.

Threshold galling stress of self-mated stainless steels on the button on block test, ASTM G98, unlubricated ground finish, reproduced from ASM Handbook, Volume 18. Entries marked (a) did not gall within the range of the test. The values are printed as they stand in the source.

What the ranking says

Among the austenitic grades the chromium manganese nitrogen alloys beat the chromium nickel alloys of similar hardness. S28200, S20161 and S21800 return the highest thresholds in the family, while N08020, the nickel rich alloy, returns the lowest at 14 MPa. The source attributes this to two alloying effects. Nickel is harmful to galling resistance and silicon is beneficial, which explains both ends of the table. Type 303, the free machining grade, sits high at 138 MPa for reasons of its own chemistry and its machined surface, and it is the useful reminder that a grade chosen for one property brings other properties with it.

Hardening an austenitic grade by cold drawing makes its galling resistance worse, not better. Type 304 falls from 55 MPa annealed at 86 HRB to 17 MPa cold drawn at 27 HRC, and S28200 falls from 166 MPa to 62 MPa over the same change. This is the single most counterintuitive line in the table, and it follows from the mechanism. Galling resistance in these alloys comes from the ability to work harden right at the contact, and a grade that has already been cold worked has spent that capacity before the part went into service.

Among the martensitic grades, higher working hardness does not by itself produce a higher threshold, and the two conditions of Type 420 show it plainly. Tempered at 204 C to 51 HRC it returns 25 MPa, and tempered at 260 C to 49 HRC it returns 55 MPa, so the softer condition is the better one. Type 440C at 55 HRC is the strongest martensitic entry at 125 MPa, which matches its reputation as the stainless tool steel for sliding contact. S42010 did not gall at 50 HRC, and the hardness figure alone would not have predicted that.

The precipitation hardening grades follow their ageing temperature rather than their alloy content. S45500 loses threshold steadily as the ageing temperature rises, from 90 MPa at 510 C to 59 MPa at 566 C and 28 MPa at 621 C, and S45000 does the same from 69 MPa to 14 MPa. A part re-aged hotter to gain ductility gives up galling resistance as it does so.

One design assumption that does not survive the data

A common working rule is that a large hardness difference between two mated parts prevents galling. The published wear data do not support it. Tested against six different counterface alloys, Type 440C at 57 HRC and S21800 at 95 HRB came out as good selections regardless of the hardness of the material they ran against. The choice of the better alloy, not the size of the hardness gap, is what decides the result. Within a single grade hardness still matters, and the same test shows Type 440C losing 0.7 mm3 at 56 HRC and 44.1 mm3 at 26.5 HRC, so the hardening treatment is not optional once the grade has been chosen.

The austenitic grades that work harden fastest are the ones that resist adhesive wear best, which is why S20161 and S21800 sit at the top of the adhesive wear ranking on the crossed cylinder test and why the martensitic Type 410 at 24.5 HRC sits at the bottom of it. The same ranking puts a plain tool steel at the front of the field.

Relative ranking of adhesive wear resistance of wrought materials, reproduced from ASM Handbook, Volume 18. Weight loss is in milligrams per thousand cycles on the crossed cylinder test, ASTM G83, at 71 N. D2 tool steel at 61 HRC is the best entry in the table, ahead of the cobalt base hardfacing alloy in the third row.

Temperature moves the answer

Adhesive wear is not a fixed property of a pair, because both the oxide film and the flow stress change with temperature. S21800 improves as the joint runs hotter, its volume loss falling from 2.6 mm3 at 80 C to 2.2 mm3 at 315 C and 1.3 mm3 at 540 C, and Type 304 holds a similar level across the same range. Type 410 behaves in the opposite way. At 95 HRB it loses 282.8 mm3 at 80 C, and hardening it to 40 HRC brings that down to 42.1 mm3, which is the case for hardening a martensitic grade even where the galling threshold says the softer temper is better. The cobalt base hardfacing alloy in the same series starts low and rises with temperature, which is the reason it is not the automatic answer at every heat.

What to do about a stainless tool that seizes

Measures against galling in stainless steel tooling and mating parts, from ASM Handbook, Volume 18. The published test methods behind the table are the button on block screening test and, for threaded connections, a make and break test on the actual connection.

Where the part is a tool steel rather than a stainless part, the same mechanism decides the grade and the answer is set out on the adhesive wear resistant grades page. The two stainless tool steels that carry this data in a tool room are 440C stainless steel, the highest threshold in the martensitic family, and 420 stainless steel, which needs the right temper to give its best result.

Grades and stock

Stainless tool steel is bought on two properties at once, and wear or galling is only the first. The corrosion side is set out on the stainless steel corrosion resistance chart, the composition side on the stainless composition chart, and the sensitization page covers what heat does to the chromium in service.

Where the damage on the part is a stain or a pit rather than a torn surface, the mechanism is different and the diagnosis is on the pitting corrosion page, with the crevice case on mold steel crevice corrosion. Where the surface tore because two parts were fretting rather than sliding, the pattern is described on the fretting wear page, and where a die gave up for a reason other than the workpiece sticking to it, the mechanism list is on the die wear and failure mechanisms page.

Source: ASM Handbook, Volume 18, Friction, Lubrication, and Wear Technology, ASM International, 1992.