Impact Wear and Spalling of Tooling
Repeated impacts wear a surface through a sequence of mechanisms, from a mild oxide film regime up to subsurface cracking that removes flakes of metal. This page collects the subsurface structure that forms under impact, the damage mechanisms and the coefficient data behind them, and the hardness and geometry rules that the spalling studies on striking tools produced.
- The model the source gives for the zero wear limit treats each impact as contributing damage from the surface shear stress and from the maximum subsurface shear stress. The material wear factor in that expression comes out at approximately 1.1 for carbon and tool steel specimens.
- Beyond the limit, the useful life of most components is tied to the loss of a specific depth of material rather than to the onset of wear, which is why the measurable wear region is the one that carries life prediction. The measured coefficient values in the table below sit in the tail of this model, where the wear volume follows the impact energy and the number of impacts.
- For unlubricated metals the sliding wear coefficient k falls between about 5 x 10 to the minus 3 and 3 x 10 to the minus 5. The impact wear coefficient K is expected to be an order of magnitude smaller, between about 5 x 10 to the minus 4 and 1 x 10 to the minus 6, and the measured values below fall inside that band.
| Surface measured | Impact energy, kgf mm | Number of impacts | Lubrication | Impact wear coefficient K |
|---|---|---|---|---|
| Copper, hard | 2 | 1 | None | 53 x 10-6 |
| Steel | 20 | 1 | None | 67 x 10-6 |
| Steel | 20 | 1 | Yes | 10 x 10-6 |
| Tungsten | 138 | 2.4 x 103 | None | 130 x 10-6 |
| Copper | 60 | 5 x 105 | None | 20 x 10-6 |
| Soft steel | 120 | 5 x 105 | None | 19 x 10-6 |
| Aluminium alloy | 60 | 5 x 105 | None | 42 x 10-6 |
| Titanium alloy | 120 | 5 x 105 | None | 15 x 10-6 |
| Hard steel | 160 | 5 x 105 | None | 28 x 10-6 |
| Hard steel | 700 | 4 x 103 | None | 43 x 10-6 |
| Hard steel | 42 | 2 x 104 | None | 104 x 10-6 |
| Hard steel | 0.36 | 6.5 x 104 | None | 14 x 10-6 |
| Hard steel | 0.36 | 4.3 x 105 | Yes | 1.2 x 10-6 |
| Soft steel | 0.36 | 1 x 104 | None | 247 x 10-6 |
| Soft steel | 0.36 | 3.2 x 106 | Yes | 0.8 x 10-6 |
Source: ASM Handbook, Vol. 11, Failure Analysis and Prevention (ASM International), impact wear coefficient values in the article Impact Wear Failures. The source table also carries a reference column, which is omitted here because its rows draw on several studies.
- Lubrication cuts the coefficient by roughly an order of magnitude on the same material at the same energy, which is visible in the two hard steel and the two soft steel rows that share an impact energy of 0.36 kgf mm.
- Soft steel at low energy and few impacts carries the highest coefficient in the set, at 247 x 10 to the minus 6. The same steel run to 3.2 million lubricated impacts falls to 0.8 x 10 to the minus 6, which is the run in behaviour that makes the first hours of a repetitive impact contact so damaging.
| Zone | What it is | What it does |
|---|---|---|
| Base material | The zone furthest from the contact. Undisturbed metal, and the reference for what the part was before it took repeated impacts. | No deformation. |
| Deformation zone | Plastically deformed base material, from no deformation at the inner boundary to a maximum at the outer boundary. Formed by the repeated stress cycle, and it work hardens as it forms. | Gradual transition into the white layer above it. |
| White layer | The surface zone, usually homogeneous and finely structured, with hardness developed in place. Its composition is often mixed, drawing material from both impacting bodies and from the environment. | Microcracks nucleate here readily once it is heavily work hardened, and the path of least resistance for them is the interface with the deformation zone. |
Source: ASM Handbook, Vol. 11, subsurface zones found beneath surfaces subjected to repetitive impact loading, in the article Impact Wear Failures.
- Hardness rises at the surface of impacting bodies. In the deformation zone this is work hardening. In the white layer the hardness is a property of the near surface material developed in place, and the layer itself forms through a combination of adhesion, mechanical mixing and diffusion.
- When the metal under the surface is brittle or does not deform, the white layer still forms and the base material stays undeformed. The layer is therefore not evidence that the part was soft or badly treated. It is evidence of repetitive impact, and in the brittle case it sits directly on undeformed metal.
- Cracks appear in some of these structures and not in others, and where they do appear they run through the white layer or along the interface with the deformation zone. Subsurface white layers have also been found at depths of tens to several hundred microns, and a crack running in one of those removes everything between it and the wear surface.
| Mechanism | What it looks like | Where it is met |
|---|---|---|
| Oxidative wear | The mild regime. Oxide films separate the bodies and cut metal to metal contact. Debris is fine and largely metal oxide. | Low stress contacts, including those with a small amount of sliding or fretting behaviour. |
| Adhesion | Material smears and transfers from one body to the other. Junctions weld at asperity tips, cracks nucleate in the work hardened tips, and the broken tips stay adhered to the other surface. | Moderate normal or compound impact, behaving much like adhesive wear in sliding. |
| Abrasion | Ploughing grooves and a roughening of the contact surface. At higher impact stresses, crack nucleation and flat plate like particles appear inside the grooves. | Mainly compound impact contacts, where sliding accompanies the impact. |
| Surface fatigue and delamination | Subsurface cracks nucleate, then grow along the white layer boundary, and material comes away as flakes. This is the spalling route. | The more severe contact conditions, where the white layer is built and work hardened. |
| Plastic deformation | Material is not removed, it is displaced out of the contact area as mushrooming. | Soft or under-hardened striking surfaces, and it acts as a warning of worse damage to come. |
Source: ASM Handbook, Vol. 11, material removal mechanisms in the article Impact Wear Failures.
| Contact condition | What happens |
|---|---|
| Low stress | Oxidative wear, small submicron particles, a highly oxidized surface. Wear rates are low because the oxide films lubricate better than bare metal. |
| Moderate impact | A process similar to adhesive wear in sliding, with smearing and transfer between the bodies. |
| More severe contact | Surface fatigue, subsurface crack growth and spalling. Tungsten carbide bits in rock drilling lose material this way. |
| Very large impact energy | Failure by surface fracture, with rapid material removal. |
Source: ASM Handbook, Vol. 11, the evolution of impact wear mechanisms as the conditions in a contact become more severe.
- On rolling contacts such as bearing races and gear teeth, pitting begins at the points of maximum Hertzian contact stress and forms small pits of about 0.5 to 1.0 mm across, with the number of stress cycles deciding how far it goes. Larger areas of material loss are called spalling, and they usually start as subsurface fatigue from cyclic rolling contact, which is why the same damage is also called rolling contact fatigue or fatigue wear.
- On striking and struck tools the same removal happens under impact instead of rolling. A spall can also follow a defect rather than a fatigue process, and one of the recorded causes is a quench crack in the tool that later opens into a spall. The way a quench crack is read before it becomes a spall is set out on the quench cracking page.
- Mechanically, the white layer work hardens until microcracks nucleate in it, and those cracks run along the path of least resistance, which is the boundary with the deformation zone beneath. Material then separates as a flake. That is a different route from the classical delamination picture, where dislocation pile up under sliding load nucleates the subsurface crack.
| Specification | Type of tool | Steel | Hardness, HRC |
|---|---|---|---|
| AREMA | Percussion track tools | 9260A, and 9260B with molybdenum | Striking face 51 to 55, struck face 44 to 48, cutting edge 56 to 60, point end 52 to 56 |
| ANSI B173.3-1991 | Heavy striking tools | 0.45 to 0.88 carbon, 0.3 to 1.2 manganese, 0.35 silicon maximum | Striking face 45 to 60, wood chopper maul 44 to 55, peen 45 to 60 |
| BS 876-1981 | Hammers | 0.5 to 0.6 carbon, 0.5 to 0.9 manganese, 0.1 to 0.4 silicon, with 0.2 to 0.3 chromium above 1.8 kg head mass | Striking face 50 to 58, and 46 at 3 mm below the face |
| GGG-H-86C-1963 | Hammers | 0.46 to 0.85 carbon, 0.2 to 0.9 manganese, 0.1 to 3 silicon | Double face 44 to 55, carpenter 50 to 60, ball peen 50 to 57, with 92 to 105 HRB under a 25 in. handle and 40 to 45 HRC over it |
| ANSI B209.3-1990 | Wood splitting wedges | Carbon steel | 35 maximum, checked with a 10 lb drop weight from 5 ft |
| ANSI B209.1-1991 | Chisels | Carbon or alloy steel | Struck face 45 maximum, cutting edge 53 to 60 |
Source: ASM Handbook, Vol. 11, striking and struck tool specifications in the article Spalling from Impact Events. Railroad striking and struck tools are currently made from grade B steel, and non percussion tools such as claw bars and rail tongs from plain carbon steel.
- The American Railway Engineering and Maintenance of Way Association specification requires the whole head of a sledge hammer or spike maul to be austenitized, quenched and tempered, rather than hardening only the striking end. The field trials in the source ran on whole head treated hammers.
- Hardness specifications for these tools bracket the safe band from below and above. Two of the six allow 60 HRC at the striking face, and the source notes that brittle cleavage fracture starts just beyond that level, which places the upper limit uncomfortably close to the safety boundary. The lower limit is about 50 HRC, reduced to about 45 HRC where the tool can strike another steel tool, because a softer face deforms over a wider area and mushrooms, which is the visible warning that a spall is coming. The reading of a hardness value on a finished tool is covered on the hardness testing methods page.
| Finding | Detail |
|---|---|
| Steel grade across a wide range | Field trials on hammers covering plain carbon steels, a 1 percent chromium molybdenum grade, a higher alloy 0.65C-1Si-3Ni grade and rim tempered hammers, all inside the specification hardness band, found no significant difference in chipping or in wear rate. Laboratory spalling tests on 1045, 1060, 1072, 1075, 1080, 1552, 9260 and 4340 steel reached the same conclusion. |
| Hardness limits that are used in practice | Two of the six specifications allow a striking face up to 60 HRC, which the source describes as very close to the safety limit, because brittle cleavage fracture begins just above it. The lower limit normally sits around 50 HRC, and drops to about 45 HRC where the tool can strike another steel tool. At 45 HRC the tool mushrooms instead of spalling, and the mushrooming is the warning. |
| Struck tool softer than the striking tool | Every specification allows the struck face a lower maximum hardness than the striking face. A softer struck tool deforms, which enlarges the contact area and spreads the load, lowering the chance of a spall in the harder tool. |
| Edge geometry | Chamfered or radiused edges raise spalling resistance for edge to surface and edge to edge impacts. A radius measured at least equal to a chamfer for small overlaps and better at a 4.8 mm overlap. The tinner setting hammers tested had no chamfer at all, and spalled readily at an angular separation of only 7.5 degrees. |
| Impact angle | For edge to surface impacts the spalling window is narrow, centred near 25 degrees. Nail hammers did not spall below about 20 degrees and spalled less often at 35 degrees. No hammer could be made to spall with a flush blow. |
| Work hardening in service | Strain hardening of a striking face during use raised the hardness by 1.5 to 2.5 points HRC in one field study. Chipping was recorded about once every 200 hammer weeks. |
Source: ASM Handbook, Vol. 11, the conclusions drawn by spalling studies and the tool geometry and impact angle sections of Spalling from Impact Events. Hammer head velocities in the laboratory tests ran from 14 m per second for driving nails to 33 m per second for deliberate spalling against a steel anvil.
Where impact wear meets tool selection
Impact loaded tooling gives up abrasive wear resistance to buy toughness, and the shock resisting grades are the ones chosen for it. The grades and their behaviour are set out on the chipping resistance page, with S7 in the S7 grade page.
When the tool cracked instead of wearing
A hard tool that fails early with no wear at all has usually gone past its toughness budget. The stage by stage causes and the examination sequence are on the tool and die failure analysis page.
Reading the failure surface
Beach marks, ratchet marks, chevrons and dimples each say something different, and the full set of readings is tabulated on the fracture surface features chart.
Separating wear from overload
Wear, fatigue and overload look different at the same magnification, and the six damage modes with their signatures at three scales are collected on the damage mode identification chart.
Confirm before quoting
Composition data is for general reference only. Actual values vary by standard, mill and heat number. Confirm against the material test certificate or contact Aobo Steel.
The coefficient values describe laboratory and field tests with the materials and contact conditions noted in the source. They size the effect of a design or lubrication change rather than specifying a life for your tool. Confirm the geometry, the hardness and the impact conditions of your own application before changing a specification.
Related reference pages
Cold work tool steels · S series shock resisting tool steels · Steel bar defects and discontinuities · Tool steel heat treatment guide · Tool steel quality verification · Tool steel supply
Source: ASM Handbook, Volume 11, Failure Analysis and Prevention (ASM International), article Impact Wear Failures by Roger Lewis and Rob Dwyer-Joyce, and article Spalling from Impact Events by Dennis L. McGarry, Gordon W. Powell and Cameron Lonsdale. Reference data for comparison only. Confirm the tool geometry and the hardness limits against the standard that governs your own product. Aobo Steel supplies tool steel in the annealed condition.