Galling

For a modeled sliding contact, 2,000 N over 100 mm² gives 20 MPa nominal pressure. Doubling area gives 10 MPa. With assumed μ = 0.20 over 50 mm, friction work is 20 J at either 10 or 5 mm/s, while average power changes from 4 to 2 W. Neither pressure nor friction work establishes a galling threshold. Inspect both mating surfaces and consider their finish, alignment and lubrication.

Galling involves adhesion and material transfer between loaded surfaces. BSSA describes junctions that form and tear in the solid state. British Stainless Steel Association, factors affecting wear and galling.

Check corrosion, damaged threads, debris and interference as other causes of seizure. Adhesive damage can occur before seizure.

Treat the contact pair as the system

Relevant inputs include both material grades and conditions, surface treatments, finish, fit, load, motion, temperature, environment and lubricant delivery. BSSA identifies design, applied load, contact area, movement, lubrication and material properties as interacting influences. High hardness alone does not establish that a pair will resist galling under the intended service. BSSA, contact and material factors.

Stainless steel fasteners are a familiar example, but the problem is not limited to them. TWI discusses stainless steel, aluminium and titanium bolts whose protective surface film can be disrupted under high-contact-force sliding. It warns that continuing to tighten seized threads can strip them or shear the bolt head. TWI, What is Galling?.

Galling is distinct from abrasive wear by entrained hard particles and from corrosion mechanisms, although contamination and environmental effects can interact with the contact. Transferred material and damage on the mating source surface may reveal different parts of the contact history.

Material selection includes corrosion and mechanical requirements

Changing one member’s alloy, hardness or surface treatment can change adhesive behavior, but it also changes the engineering system. ASSDA discusses pairing austenitic stainless bolts with other nut materials and stresses the corrosion implications of alternatives. Its guidance also notes that some surface-hardening treatments can reduce the corrosion resistance of stainless steel. A harder nut or coating may also affect strength, temperature suitability, dimensional fit and compliance with the governing specification. ASSDA, FAQ 5: Galling and its Control.

A named producer example is Carpenter’s Gall-Tough stainless, UNS S20161. Its 2009 alloy-selection guide describes a high-silicon, high-manganese, nitrogen-strengthened austenitic alloy with self-mated galling and metal-to-metal wear resistance, and identifies potential moving-component applications. This is a specific alloy option, not a guarantee of galling-free service or an automatic substitute for a specified corrosion-resistant grade. Carpenter Technology, 2009 alloy-selection guide, Gall-Tough entry.

A test result depends on the material pair and treatment condition. A result for self-mated material does not establish behavior against a different counterface.

Lubrication and assembly are controlled choices

TWI recommends considering thread lubrication, reduced tightening speed and suitable dissimilar mating materials. It explicitly notes application restrictions on lubricant chemistry, including food-industry use, and corrosion concerns with unlike alloys. These are candidate controls, not permission to apply any anti-seize compound to every assembly. TWI, prevention guidance.

For a threaded joint, the tightening basis depends on the actual surface and lubrication condition. ASSDA emphasizes torque appropriate to grade, size, pitch and lubrication, and notes risks from both over- and undertightening. Changing friction while retaining a torque value qualified for another condition can invalidate the assembly basis. Reaching a torque setting alone does not establish clamp load or satisfactory assembly. ASSDA, surface contact stress and assembly guidance.

Thread condition and fit, lubricant identity and application, temperature, environment and reuse can affect assembly performance. A lubricated test does not establish dry-running capability; a fresh assembly does not establish repeat-disassembly performance.

Interpret galling tests within their boundaries

ASTM G196-24 describes a method for ranking material couples against galling failure, rather than merely classifying the appearance of sliding surfaces. Its public significance-and-use text distinguishes it from G98 initial ranking and discusses slow, intermittent sliding systems. The method and specimen conditions limit interpretation of a reported result. ASTM G196-24, public scope and significance.

ASSDA’s example test table uses rotating pin-on-block experiments and reports a threshold stress for particular material pairs. Such a test result is not a universal allowable bearing pressure, bolt preload or tightening torque. The real contact geometry, finish, lubrication, motion and environment can differ materially from the test. ASSDA, experimental-evidence discussion.

Interpretation depends on the test method and edition, materials and conditions, counterface, preparation, lubricant, load sequence and definition of failure. A single ranking cannot demonstrate complete component life, corrosion performance or compliance with an application-specific acceptance requirement.

Original example: nominal pressure and friction work

Assume an invented flat sliding contact carrying 2,000 N over a nominal 100 mm² area. Its nominal pressure is:

p = F/A = 2,000/100 = 20 N/mm² = 20 MPa.

Doubling that nominal area to 200 mm² at the same load reduces the calculated nominal pressure to 10 MPa. This arithmetic does not describe local asperity pressures, edge loading or the distribution created by misalignment. Neither result is a galling threshold or an allowable design value.

For a separate simplified friction calculation, assume a constant coefficient μ = 0.20 and a sliding distance of 50 mm. Friction force is μF = 400 N, and work is 400 × 0.050 = 20 J. These are hypothetical model inputs, not measured properties of stainless steel or a lubricant.

At 10 mm/s, the travel lasts five seconds and average friction power is 4 W. At 5 mm/s, it lasts ten seconds and average power is 2 W, while total work remains 20 J. This illustrates why speed, power and total friction work are different quantities. It does not predict temperature or galling: heat partition, cooling, changing friction, surface damage and contact mechanics are omitted. The calculation supplies no safe assembly speed.

Diagnosis and acceptance limits

Resistance or damage can appear during first assembly, reuse, loaded service or disassembly. The mating materials and treatments, contact dimensions, lubricant, motion and loading history all affect interpretation. Additional torque or aggressive cleaning does not by itself prove recovery of a damaged assembly.

Evidence from nonrepresentative mating parts, environments or motion histories may not predict the intended application. Damage to a counterface may remain after replacement of a seized member. A laboratory ranking alone does not establish functional dimensions, assembly performance or application-specific acceptance.

Source versions

Reviewed 2026-11-02 PST. The cited ASTM record identifies G196-24; this page does not assert that it supplies application-specific allowable stresses. Carpenter’s linked guide identifies its 2009 edition. No separately verified issue date is assigned to the BSSA, ASSDA or TWI web guidance. PDF filenames, crawl dates and hosting paths are not treated as editions. Numerical examples are original and bounded to their assumptions.