NingBo,Zhejiang, China-September 14,2026

For a transmission tower foundation on a granite ridgeline, for a wind turbine anchor cage on a weathered basalt slope, or for a road-cut retaining wall that has to survive a 50-year service life, the M16-M36 anchor bolt is not just a fastener. It is the load path between the structure and the rock. The forming process, the surface hardness, and the inspection regime all decide whether that bolt will hold for the design life or fail prematurely at the rod-to-grout interface.

Grouted Rock Anchor (Weiye Corp) — 1.25 in (31.7 mm) steel rod, 3 ft (914 mm) length, HDG finish. Source: xyweiye.com product page.

This article walks through the actual process flow that Weiye Corp uses to build M16-M36 anchor bolts for grouted rock anchor applications, drawing on the same Grouted Rock Anchor product line. The flow is a 6-stage multi-station cold heading sequence, induction surface hardening to the HRC 35-42 range, and 100% eddy current flaw detection before hot-dip galvanizing. The aim is to give procurement engineers, quality managers, and sourcing teams a single document they can pin to the inside of a procurement folder and use to evaluate a cold heading supplier against a project specification.

1. Why the M16-M36 Anchor Bolt Needs a 6-Station Cold Heading Flow

At M16 and above, the bolt geometry is no longer something a single die blow can produce. The head, the shank, the unthreaded run-out, and the thread-rolled end each require their own forming event, and the medium-Carbon Steel used for anchor bolts (typically 1038, 1045, or 4140 grade per ASTM F1554 chemistry) has a finite amount of ductility before work-hardening cracks start to appear. Multi-station cold heading solves both problems by breaking the deformation into smaller, controlled events.

Cold heading is a cold forming process where a metal blank is forced into a die cavity at room temperature. The ram and punch deform the blank plastically without heating, which means the steel’s grain structure stays intact and the Surface Finish is smoother than hot forging. Each station in a multi-station press performs one specific operation – extrude, upset, trim, pierce, finish, or sizing – and the blank transfers from station to station inside the press. By the time it reaches the last die, the bolt has the finished head, the correct shank diameter, and a smooth transition geometry ready for thread rolling.

For M16-M36 anchor bolts going into grouted rock anchors, six stations is the typical minimum because the cross-section reduction from the bar stock to the shank is significant. Trying to do it in fewer stations means a higher percentage reduction per blow, which raises the die load, increases the risk of laps and bursts, and limits the maximum length that can be produced. Six stages give the press enough rope to form a long-shank anchor without exceeding the ductility of the steel in any single blow.

Station 1ExtrudeStation 2Upset HeadStation 3Trim FlashStation 4Pierce / ConeStation 5Finish FormStation 6Sizing6-Station Multi-Station Cold Heading Sequence (M16-M36)Bar blank enters at left; finished headed bolt exits at right1.1 How 6 stations map to M16-M36 anchor bolts

The station count is not arbitrary. For the M16-M36 anchor bolt range that Weiye Corp produces for grouted rock anchor and transmission foundation programs, six stations map roughly as follows:

• Station 1 – Extrude: Forward extrusion reduces the bar cross-section to the final shank diameter. This is the highest reduction event and is usually run at the slowest ram speed of the cycle.
• Station 2 – Upset Head: The upset operation gathers material at one end and forms the bolt head geometry. For hex or heavy hex heads, the upset cavity is sized to the final head dimensions with a small draft allowance for the trim station that follows.
• Station 3 – Trim Flash: Any excess material that flowed into the die parting line during upset is trimmed off. This station is what gives the bolt its clean head-to-shank transition.
• Station 4 – Pierce / Cone: For headed anchor bolts that need a forged point or cone on the unthreaded end (common in rock anchor applications), a piercing or cone-forming station shapes the leading geometry. If no point is required, this station is configured as a sizing pass.
• Station 5 – Finish Form: The final cold forming pass refines the head dimensions and the shank diameter to within the drawing tolerance, leaving a controlled amount of material for the thread rolling that comes later.
• Station 6 – Sizing: A light final sizing pass brings the shank and head to the final dimensional specification. This station also serves as a visual quality gate because any die wear or material defect shows up as a dimensional deviation here.

Process engineers will recognize this as a fairly conventional multi-station cold heading sequence; the variation Weiye Corp applies for grouted rock anchor bolts is the choice of medium-carbon steel grade and the post-heading inspection regime, both of which are discussed below.

2. Steel Grade and Dimensional Window for the M16-M36 Range

The M16-M36 anchor bolt window is wider than it looks. M16 (5/8″) is on the lower edge of what most cold heading presses want to run efficiently, while M36 (1-7/16″) sits at the upper edge of what can be cold headed without going to a larger press frame. Inside that window, the choice of steel grade drives both the forming sequence and the achievable hardness range after induction hardening.

Diameter (Metric)	Diameter (Imperial)	Common Steel Grades	Typical Application
M16	5/8″	1038 / 1045 medium-carbon	Light rock anchor, cable stay anchor
M20	3/4″	1038 / 1045 / 4140	Transmission tower foundation anchor
M24	1″	1045 / 4140	Wind turbine anchor cage
M30	1-3/16″	4140 / 4140 mod	Heavy retaining wall anchor
M36	1-7/16″	4140 / 4140 mod	Bridge pier anchor, dam foundation

Weiye Corp sources these grades to ASTM F1554 chemistry and mechanical property ranges for the structural anchor grades, with a chemistry variation window that the customer can override on the drawing. For grouted rock anchor bolts the typical supply is to F1554 Grade 36 or Grade 55, with Grade 105 used where the customer specifies higher tensile strength.

Note on diameter: The dimensional window above is the range Weiye Corp currently produces on its 6-station cold heading presses. M16 and M20 share die sets with the wider fastener production; M24 through M36 run on dedicated larger-diameter tooling.3. Why Induction Hardening Lands at HRC 35-42 for Anchor Bolts

Through-hardening a medium-carbon steel bolt to HRC 35-42 would make the entire cross-section brittle, which is the opposite of what a grouted rock anchor bolt needs. The bolt has to be hard on the outside (where installation abrasion, grout placement, and in-service wear happen) and tough on the inside (where tensile load, cyclic stress, and impact have to be absorbed without cracking). Induction hardening is what makes that combination possible.

Induction hardening uses an alternating magnetic field from a copper coil to heat only a thin surface layer of the steel – typically 1.5 to 3 mm deep for the M16-M36 anchor bolt range – then quenches that layer with water or polymer.The surface heating profile (case depth in mm, as-quenched surface hardness in HRC, temper temperature in deg C) is tuned per alloy and per lot. Weiye Corp runs the heat treatment step on an in-house induction line tied to the cold heading presses, so the case hardness map travels on the same lot traveler as the dimensional and ET data.

The heated surface layer transforms to martensite on quenching and lands in the HRC 35-42 range, while the core of the bolt stays at its original annealed or normalized hardness. The result is a hard case on a tough core, with no need to reheat the entire bolt.

Why HRC 35-42 specifically? It is a deliberate compromise between three competing requirements:

1. Abrasion resistance: At HRC 35-42 the surface is hard enough to resist scoring and wear from installation tools, the grout mixture, and any backfill material that contacts the bolt.
2. Toughness reserve: Below HRC 42, the core stays tough enough to absorb the cyclic and impact loads that grouted rock anchors see during seismic events, thermal cycling, and rock-mass movement.
3. Galvanizing compatibility: Hot-dip galvanizing at around 450 deg C is the last process the bolt sees. A case hardness below HRC 42 means the zinc bath will not temper the hardened layer back to unacceptable softness.

The relationship between case hardness, case depth, and core hardness is what induction hardening equipment makers tune on a per-alloy basis. For Weiye Corp’s typical 4140 anchor bolt the as-quenched surface hardness lands in the HRC 50-55 range; the post-temper hardness target is the HRC 35-42 window above. Tempering temperature and time are recorded on the lot traveler so the customer can audit it.

The “as-quenched HRC 50-55 then temper down to HRC 35-42” pattern is consistent with published induction tempering practice for medium-carbon alloy steels: temper just below the temperature that would push the case hardness below the abrasion resistance floor.

For procurement, a project quality plan should specify the as-quenched surface hardness range, the case depth in mm measured to a defined hardness transition, and the core hardness measured at mid-radius. Weiye Corp releases these three numbers per lot in the project’s PPAP or FAI package.

4. 100% Eddy Current Flaw Detection – What It Catches and What It Doesn’t

Cold heading is a forgiving process compared to machining, but it is not defect-free. The two defect families that show up most often are longitudinal cracks (typically at the head-to-shank fillet) and laps (small surface folds where material flowed into a die cavity and folded over itself). Both are surface or near-surface defects, and both are exactly what eddy current testing (ET) is built to find.

Eddy current testing is an electromagnetic nondestructive method that uses a coil to induce alternating currents in the conductive surface of the part being inspected. Where the surface is clean and uniform, the eddy current flow is also uniform and the inspection coil sees a stable baseline signal. Where there is a crack, lap, seam, or abrupt dimensional change, the eddy current flow is disturbed and the inspection coil picks up a signal change. The ET method is sensitive to surface and near-surface defects within roughly 1-3 mm of the surface depending on the test frequency, the coil diameter, and the material conductivity – which is exactly the depth range where cold heading defects live.

AC CoilTest FrequencyAnchor Bolt (M16-M36)Signal AnalysisPass / RejectEddy Current Flaw Detection: Induced current disturbed by surface/near-surface defectSensitivity: 1-3 mm depth range. Defects caught: cracks, laps, seams, abrupt dimensional changes.

For M16-M36 anchor bolts the typical ET setup is a dual-coil encircling arrangement where the bolt passes through the inspection head in a continuous feed. The coil is tuned to a reference standard that contains artificial defects of defined size, and the acceptance threshold is set so that any natural defect equal to or larger than the reference defect triggers a reject signal. 100% ET means every bolt in the lot passes through the inspection head, not a sampled subset. ASTM E243 and E309 cover the underlying ET practice for ferrous products, and Weiye Corp applies equivalent practice to solid anchor bolts, with the calibration standard tailored to the bolt diameter and the acceptance criteria.

What ET does not catch is also worth being clear about. Subsurface forging porosity deeper than the penetration depth of the test frequency will not show up on ET. For that, ultrasonic testing (UT) is the right tool. Weiye Corp uses ET for 100% inline inspection of every M16-M36 anchor bolt in the lot because the defect family of interest is at the surface, and uses UT as a sampled check on first article and on process changes.

4.1 Where 100% ET fits in the lot release sequence

1. Lot released from cold heading with dimensional and visual inspection complete.
2. Lot induction hardened; temper pass complete; case hardness and depth recorded.
3. 100% ET inspection – reject signals flagged, parts removed from the lot, lot traveler annotated.
4. Thread rolling (if not already part of the cold heading sequence).
5. Hot-dip galvanizing to ASTM A-153 or A-153M (or as customer specified).
6. Final dimensional and visual inspection.
7. PPAP or FAI documentation compiled.

The position of ET before galvanizing is deliberate. Galvanizing hides surface defects under a zinc layer that is itself non-uniform; an ET scan after galvanizing produces signals that mix zinc thickness variation with real defects, which raises the false reject rate. Scanning before galvanizing, on the bright cold-headed and induction-hardened surface, gives a clean defect signal and a clean pass / reject decision.

5. Hot-Dip Galvanizing and Final Acceptance

For grouted rock anchor bolts that see soil, groundwater, and in some cases coastal or sulfate-rich environments, hot-dip galvanizing to ASTM A-153 or A-153M is the standard corrosion protection, and Weiye Corp supplies a galvanized anchor rod matched to each grouted rock anchor configuration.

The zinc coating weight and the coating thickness are recorded per heat lot on the galvanized certificate. Weiye Corp also runs an in-house anchor bolt production line that covers the full chain from cold heading through galvanizing, so the ET inspection, induction hardening, and galvanizing happen in one factory flow rather than three different subcontractors.

Final acceptance is a documentation pass rather than a physical inspection pass. The lot is paired with a mill test report (MTR) for the steel grade used, including the heat number and the chemical and mechanical results; a hardness map for the induction case, recorded at the head-to-shank fillet, mid-shank, and thread-rolled end positions; an ET acceptance log with the calibration standard reference and the pass / reject count; a galvanizing certificate with the zinc coating weight per ASTM A-153 or A-153M; and a dimensional report covering the bolt geometry against the customer drawing.

6. Sourcing Checklist for M16-M36 Anchor Bolt Programs

For procurement and quality teams evaluating a cold heading supplier against a grouted rock anchor bolt specification, the following 10-point checklist collapses the conversation into the items that decide whether the bolt will perform in service. Bring it to the supplier audit, walk through it line by line, and use the answers as the basis for either approving the supplier or asking for a process change before the first production lot ships.

1. Steel grade and F1554 class on MTR: Confirm the supplied grade matches the drawing and the project specification (F1554 Grade 36, 55, or 105). Ask for the heat number and a copy of the MTR.
2. Cold heading station count for the M size: Confirm that the supplier runs the M16-M36 anchor bolt range on a 6-station press, and confirm the largest single-blow reduction as a percentage of bar area.
3. Case depth and case hardness target: Ask for the as-quenched surface hardness, the post-temper surface hardness target (typically HRC 35-42), and the case depth in mm measured to the HRC 35 transition.
4. Core hardness measurement position: Confirm that core hardness is measured at mid-radius, not at the centerline, so the value reflects the worst-case toughness of the bolt.
5. 100% ET inspection scope: Confirm that 100% ET is applied to every bolt in the lot, not a sampled subset, and ask for the calibration standard reference defect size.
6. ET position in the process flow: Confirm that ET runs before hot-dip galvanizing, not after, so the zinc coating does not contaminate the inspection signal.
7. Galvanizing certificate to A-153 or A-153M: Confirm the zinc coating weight (typically 1.7 oz/ft^2 minimum for anchor bolts) and the coating thickness range on the lot certificate.

Hardness map report content:

Confirm the hardness map covers at least the head-to-shank fillet, mid-shank, and thread-rolled end positions.
1. Traceability record per lot: Confirm that each production lot is traceable to a specific coil or bar bundle, with the heat number, the production date, and the inspection records linked.
2. FAI / PPAP package content: Confirm the FAI or PPAP release package includes the MTR, the dimensional report, the hardness map, the ET log, the galvanizing certificate, and the traceability summary.

None of these ten items requires special equipment to check. They are document reviews and short conversations. A supplier that can answer all ten cleanly is far more likely to ship bolts that survive in service than a supplier that can only answer two or three.

7. Frequently Asked Questions7.1 What is multi-station cold heading for anchor bolts?

Multi-station cold heading is a cold forming process where a metal blank passes through several die stations in sequence inside a single press. Each station performs one forming operation – extrude, upset, trim, pierce, finish, or sizing – so the bolt shape is built up progressively rather than in a single blow. For M16-M36 anchor bolts, a 6-stage sequence gives the press enough stations to form the head, the shank diameter transitions, and the lead-in geometry without exceeding the ductility of the medium-carbon steel in any single blow.

7.2 Why use induction hardening instead of through-hardening for anchor bolts?

Induction hardening heats only a shallow surface layer of the bolt – typically 1.5 to 3 mm deep – then quenches that layer so it reaches the target hardness while the core stays tough. For grouted rock anchors this matters because the surface has to resist abrasion and wear during installation and grout placement, while the core has to absorb impact and cyclic loads without cracking. Through-hardening makes the whole section brittle and is unsuitable for the M16-M36 anchor bolt range where tensile ductility and impact toughness are still required.

7.3 What does 100% eddy current flaw detection actually catch?

Eddy current testing (ET) is an electromagnetic method that detects surface and near-surface defects on electrically conductive parts. On M16-M36 anchor bolts, 100% ET inspection is used to find longitudinal cracks, laps, and seams from the cold heading process that would otherwise be hidden under the galvanized or HDG coating. ET is sensitive to defects within roughly 1-3 mm of the surface depending on the test frequency, which matches the depth range where cold heading cracks and forging laps occur most often.

7.4 Does Weiye Corp publish the M16-M36 dimensional tolerances and hardness depth?

Weiye Corp manufactures M16-M36 anchor bolts to customer drawing and to referenced standards such as ASTM F1554 Grades 36, 55, and 105. Specific dimensional tolerances, induction case depth in mm, and ET sensitivity thresholds are released with the project-specific PPAP package. The 100% ET acceptance criteria, induction hardening target hardness range, and galvanized coating weight per ASTM A-153 or A-153M are documented in the project quality plan before production starts.

7.5 What is the difference between cold heading and hot forging for anchor bolts?

Cold heading forms the bolt at room temperature inside a multi-station press, which gives high dimensional accuracy and a smooth as-formed surface finish but limits the cross-section reduction per pass. Hot forging heats the bar above the recrystallization temperature and forms it in a die under hammer or press load. Weiye Corp uses cold heading for the M16-M36 range where the section reductions are within the work-hardening capacity of medium-carbon steel, and switches to hot forging for larger sections or for parts where the grain-flow pattern from forging is specified by the customer.

7.6 Can Weiye Corp supply a full PPAP or FAI package for M16-M36 anchor bolts?

Yes. For grouted rock anchor and transmission tower foundation programs, Weiye Corp releases a project quality plan covering raw material certificates, dimensional reports, hardness mapping of the induction case, ET acceptance data, hot-dip galvanizing certificate to ASTM A-153 or A-153M, and a 100% traceability record linking each production lot to the coil it came from. The package is sized to the customer’s drawing and to the referenced standard.

Send Your M16-M36 Anchor Bolt Drawing for a Process Audit

If you have a grouted rock anchor or transmission foundation bolt drawing – M16 to M36, F1554 grade, induction case depth requirement – send it to the request cold heading process audit. We will respond with a process flow showing how the bolt will be cold headed, induction hardened, eddy current tested, and hot-dip galvanized, plus a sample FAI package format for your quality team to review before the first production lot ships.

About the Weiye Corp Engineering Team

The Weiye Corp engineering team writes about how structural fasteners and anchor bolts are actually made at the Weiye factory – from bar stock to packaged pallet. The team has spent the last decade working with cold heading presses, induction hardening coils, eddy current inspection heads, and hot-dip galvanizing baths, applying that hands-on knowledge to the M16-M36 anchor bolt, the Expanding Socket Rock Anchor, and the related hot-forged rock anchor product lines. Articles are based on the factory floor data and customer project quality plans, not on marketing claims.

About us

Weiye Corp, an innovative and dynamic company with over three decades of experience in the manufacturing sector. Established in 1990, we initially specialized in fastener manufacturing but have since expanded our capabilities to encompass hot forging, cold heading, stamping, machining, heat treatment, and material testing.

Media Contact
Company Name: Ningbo Xinyi Weiye Import & Export Co., Ltd.
Contact Person: Media Relations
Email: Send Email
Country: China
Website: https://www.xyweiye.com/