How to produce A193 B7 stud bolts?

Complete Production Process of ASTM A193 B7 Stud Bolts

A193 B7 is quenched & tempered 4140 chrome-moly alloy steel stud bolt for high-temperature/high-pressure flanges, matched with A194 2H nuts. The full manufacturing workflow complies strictly with ASTM A193/A193M standard.

1. Raw Material Inspection & Preparation

Base Material

• Standard raw material: AISI 4140 / 4142 Cr-Mo alloy steel round bar (0.38–0.48% C, 0.80–1.10% Cr, 0.15–0.25% Mo)

• Steelmaking route: Electric furnace / basic oxygen furnace, vacuum degassing optional for critical service

Incoming QC

1. Review mill test certificate (MTC): chemical composition, raw hardness, grain structure

2. Visual & dimensional check of round bars; remove surface cracks, rust, decarburization

3. Cut bars to blank length by band saw / cutting machine, leave machining allowance

2. Blank Forming (2 production routes)

Route A: Cold Forming (M2–M24 small studs)

1. Cold straightening: eliminate bar bending

2. Cold heading / upsetting: form smooth central shank or transition shoulder between two threaded ends

3. Shot blasting to remove cold forming oxide skin

Route B: Hot Forging (M24–M100 large heavy-duty studs)

1. Heat blanks to 850–950°C in furnace

2. Hydraulic press for hot forging to shape shank; refine metal grain flow for higher toughness

3. Air cool + descaling blasting

3. Precision Machining & Thread Processing

Two mainstream thread manufacturing methods:

Method 1: Thread Rolling (Preferred, high fatigue resistance)

1. Machine outer diameter to thread pitch diameter tolerance

2. Roll threads with cylindrical thread dies (cold displacement, no material cutting)

3. Chamfer both ends: 30–45° bevel to avoid thread galling during nut assembly

4. Stamp marking on one end: grade “B7” + manufacturer identifier (mandatory per ASTM A193 B7)

Method 2: Thread Turning (Custom special-length / non-standard studs)

• CNC lathe cut threads for oversized, special undercut shank studs; lower fatigue performance vs rolled thread

4. Critical Heat Treatment (Core Step for B7 Mechanical Properties)

ASTM mandatory rule: Quench + full temper liquid heat treatment to hit strength targets

1. Austenitizing: Heat blanks to 840–870°C, hold for uniform heating

2. Liquid quenching: Rapid cool in oil / polymer to form martensite

3. Tempering (minimum 593°C / 1100°F holding):

◦ Tempering temp ≥593°C for all B7 sizes

◦ Hold enough time then air cool to room temperature

Standard Mechanical Targets (post heat treatment)

• Min tensile strength: 860 MPa (125 ksi)

• Min yield strength: 720 MPa (100 ksi)

• Max hardness: 321 HBW / 35 HRC

• Elongation ≥16%, reduction of area ≥50%
Note: For B7M variant (sour gas service), temper at ≥620°C, 100% hardness sorting required; standard B7 does not need full 100% hardness inspection.
5. Post Heat Treatment Finishing

1. Stress-relief blasting: remove oxidation scale from heat treatment

2. Straightening: correct minor bending after quenching

3. Thread re-cleaning: brush or light grind to clear quenching residue in thread roots

6. Surface Treatment (Select per service environment)

1. Black oxide (standard bare finish): Low-cost, temporary anti-rust for indoor high-temp piping

2. Hot-dip galvanizing: Outdoor atmospheric corrosion resistance (not for >300°C service)

3. PTFE / Xylan coating: Anti-galling, low friction for frequent disassembly flanges

4. Dacromet: Thin coating, no hydrogen embrittlement risk

7. Strict Final Quality Control (Full lot inspection)

1. Dimensional inspection: thread pitch, major/minor diameter, overall length, chamfer

2. Hardness test: random sampling Rockwell/HBW hardness check

3. Tensile test: destructive tension test to verify yield & tensile strength

4. NDT optional: Magnetic particle inspection (MPI) to detect surface cracks for critical pressure vessel studs

5. Thread fit test: match with standard A194 2H nuts for smooth engagement

6. Marking recheck: confirm B7 grade stamp is clear and legible

8. Packaging & Delivery

• Pack into moisture-proof plastic bags + wooden cases

• Attach batch MTC including heat number, chemical analysis, mechanical test data, heat treatment records

Shortcut Mass Production for Fully Threaded Studs

For full-length all-thread B7 studs (no smooth shank):

1. Heat treat full-length 3–4m 4140 round bar first

2. Roll continuous threads on heat-treated bar

3. Cut to customer required stud length, chamfer ends, stamp marking, surface coat, inspect