Why Simply Scaling Up Does Not Work

When the design lacks load capacity, the most direct thought is "scale M24 up to M48, the cross-sectional area quadruples, problem solved." But when you actually do it, you find that: even both labeled grade 10.9, the actual performance of an M48 bolt may differ greatly from an M24.

The reason is that scaling up brings more than just a change in cross-sectional area: it includes a drop in core cooling rate during quenching, an increased probability of internal material defects, a changed residual stress distribution, and greater difficulty in hydrogen absorption and relief during surface treatment. Together these factors mean large-size bolts require an independent set of standards and evaluation methods—this is the purpose of GB/T 3098.23-2020 (M42–M72 bolts, screws, and studs).

What GB/T 3098.23 Covers

GB/T 3098.23 addresses thread sizes from M42 to M72 and specifies the following:

It specifies the applicable property classes and material requirements (mainly grades 8.8 and 10.9; grade 12.9 is rarely used in large sizes), the mechanical property indicators (tensile strength, yield strength, elongation, reduction of area, impact absorbed energy, etc.), test methods, and the applicable size range. Compared with GB/T 3098.1, the most notable differences are three: an explicit requirement for impact toughness (judged by temperature and class, not just tensile properties); a defined specimen sampling location (core or specified position); and an added core hardness requirement. The reason is simple—large-size bolts are typically used in heavy structures, wind power, bridges, and large pressure vessels under dynamic load, where insufficient toughness means catastrophic consequences.

Manufacturing Challenge 1: Insufficient Hardenability

This is the most central technical challenge for large-size bolts.

Physical Cause

During quenching, the coolant only contacts the surface; heat must conduct from the core to the surface before being carried away. The larger the size, the longer the core-to-surface distance and the slower the cooling. When the core cooling rate falls below the material's critical cooling rate, the core cannot form martensite and only yields low-strength structures such as pearlite and bainite. The result is that the surface hardness meets spec while the core is low, forming an uneven "hard surface, soft core" state, which in tensile testing manifests as insufficient overall strength or a mixed fracture morphology.

Three countermeasures: choose materials with better hardenability (adding Cr, Mo, Ni, such as 42CrMo, 40CrNiMo; Ni-bearing steel has markedly better hardenability); increase quench severity (use agitated water-based or polymer quenchant instead of oil quenching); and match the upper size limit to the grade—some materials can reach grade 10.9 at M42 but not at M72, where they must drop to grade 8.8; this is determined by material capability, not something process effort can overcome.

Manufacturing Challenge 2: Amplified Hydrogen Embrittlement Risk in Surface Treatment

Large-size bolts are mostly used in heavy-load structures, typically grade 10.9 with tensile strength above 1000 MPa, which is in the hydrogen-embrittlement-sensitive range. And plating large parts is technically more difficult:

Three amplifying effects: uneven current distribution (large parts have big surface areas, with clear current-density differences between edges and center, uneven coating thickness, and possible local porosity or stress concentration); low hydrogen-relief efficiency (hydrogen atoms must diffuse from the surface to the core and then out; the path is long, so the same relief time is far less effective than for small parts); and extended pickling time (descaling requires longer pickling, increasing hydrogen absorption).

The countermeasure has five points: switch to non-plating processes (zinc-aluminum coating/Dacromet, mechanical zinc plating, fundamentally avoiding hydrogen absorption); extend hydrogen-relief time (to 8–24 h depending on size and strength class); control relief temperature (usually 190–230°C, must be below the tempering temperature); relief promptly (start within 1–4 h after plating); and verify per GB/T 3098.17 with a pre-load test to check for delayed fracture.

Manufacturing Challenge 3: Testing and Inspection Capacity

Inspecting large-size bolts is itself an engineering problem, and the capability gaps in many inspection stages are more intractable than manufacturing itself.

Four gaps in inspection capacity: the minimum tensile load test can reach thousands of kN, requiring a ≥3000 kN large-capacity tensile machine; the wedge load test requires specially made large-size fixtures and wedges, with concentricity verified; the impact test requires specimens cut from the bolt at defined positions, with a Charpy impact tester and low-temperature chamber; decarburized layer and metallography involve large cross-sections, time-consuming specimen preparation and observation, requiring a metallographic microscope and standard hardness blocks; and thread inspection large-diameter gauges are expensive and bulky, so coordinate measuring machines are usually used instead.

Taking an M72 grade 10.9 bolt as an example, its stress cross-sectional area is about 3245 mm²; using the lower tensile strength of 1040 MPa, the minimum tensile load is about 3370 kN (about 344 t). This means the tester's capacity reserve must have margin, and the fixture itself must withstand this load level without introducing off-center loading.

Practical Advice on Selection and Acceptance

Selection Stage

The selection stage has four points: first confirm the largest size the material can cover—do not jump to a size just because of the strength class; first ask the supplier "how large can this class be made reliably"; calculate the engagement length—large sizes are usually taken at 0.8d–1.0d, with M48 corresponding to about 40–48 mm, imposing clear requirements on nut height or tapped-hole depth; confirm tightening equipment capacity—tightening torque can reach thousands of N·m, requiring hydraulic tensioners or dedicated torque equipment; ordinary torque wrenches cannot meet the design requirements; and evaluate alternatives—above M42, also evaluate hydraulic tensioning, multiple smaller bolts in parallel, or interference fits, which are sometimes more economical and reliable than a single oversized bolt.

Acceptance Stage

The acceptance stage should cover seven items: chemical composition matches the standard, checking C, Mn, P, S, and alloy elements; surface and core hardness both meet spec, with core-to-surface hardness difference ≤40 HV; hardness gradient transitions evenly along the cross-section with no abnormal soft bands; impact toughness measured at the corresponding class and temperature, not replaced by tensile data; decarburized layer meets the standard's depth limits; hydrogen embrittlement verification per GB/T 3098.17 with a pre-load test, showing no delayed fracture; and thread precision with the GO gauge passing and the NO-GO gauge not exceeding the specified engagement; in addition, marking, batch number, and heat number must be complete and traceable.

Conclusion: Large Size Is a Capability Issue, Not a Size Issue

The technical difficulties of M42–M72 bolts all stem essentially from "how physical laws change when size is scaled up": longer cooling paths cause insufficient hardenability, longer diffusion paths make hydrogen relief difficult, and rising load magnitudes limit testing and assembly equipment. Understanding these three points explains why large-size bolts cannot simply be accepted per GB/T 3098.1, and require independent standards and methods.

For engineering selection, the most pragmatic approach is to engage early, before the size is fixed, with a supplier that has large-size manufacturing and inspection capability, and settle in one discussion the achievable material class, the achievable upper size limit, and the inspection plan. The cost of trial and error in large-size joints is extremely high—one non-conforming batch may mean delayed delivery of an entire piece of equipment.