Bolt & Nut Annealing Q&A | Spheroidizing Annealing Explained | QBH Fastener

Introduction
In fastener manufacturing, annealing is a heat treatment process that often goes unnoticed but is critically important. Many procurement and quality control professionals focus only on the final hardness and strength, overlooking the decisive role annealing plays in material plasticity, internal structure, and subsequent processability. In this article, we answer five frequently asked questions about bolt and nut annealing from a practical production perspective, helping you understand why high‑quality fasteners depend on a proper annealing process.

What is annealing, and why is it used in bolt and nut production?

Annealing is a heat treatment process in which metal is heated to a certain temperature (usually above the recrystallization temperature), held there for a period, and then slowly cooled. Its main purposes are to reduce hardness, eliminate internal stress, improve structural uniformity, and increase plasticity.

In bolt and nut production, annealing is used in several stages:

1. Wire annealing before cold heading (spheroidizing annealing)
   Cold heading requires the wire to have high plasticity. If the wire is too hard, it may crack during cold heading or cause excessive die wear. Spheroidizing annealing makes the carbides inside the wire spheroidal, significantly reducing deformation resistance.

2. Intermediate annealing after work hardening
   For complex parts that require multiple passes of cold drawing or cold forming (e.g., special‑shaped nuts, long bolts), the material becomes brittle due to work hardening. Intermediate annealing restores plasticity so that forming can continue.

3. Residual stress relief
   After cold heading, cold extrusion, or machining, internal residual stresses exist in the part. If not removed, they may cause deformation or cracking during subsequent heat treatment (quenching) or in service.

Real‑world case:

An automotive fastener supplier experienced batch cracking in the heads of M12 flange bolts during cold heading. Analysis showed the wire rod supplied had not been properly spheroidized – the pearlite structure was coarse and lamellar. We recommended adding one cycle of spheroidizing annealing at 740°C. The cracking rate dropped from 12% to 0.3%.

What are the common types of annealing? Which is most often used for bolts and nuts?

Several types of annealing exist. The most common in the fastener industry are:

For bolts and nuts:

• Cold‑heading wire (e.g., 10B21, 35K, 40Cr, SCM435) → Spheroidizing annealing is most common. Spheroidization grade ≥ 4 (according to relevant standards) is required.

• Intermediate treatment after work hardening → Use recrystallization annealing or stress relief annealing.

How do you judge whether annealing quality is acceptable? What are the inspection criteria?

Annealing quality cannot be judged by hardness alone; microstructure and process parameters must also be considered. Professional suppliers typically check the following three items:

1. Hardness test

   • After spheroidizing annealing, wire hardness is typically HRB 70–85 (varies slightly by steel grade).

   • Too high → insufficient plasticity, risk of cracking during cold heading.

   • Too low → possible overheating or decarburization.

2. Spheroidization grade

   • Evaluated under a metallurgical microscope according to standards such as GB/T 38770 or SEP 1520.

   • For fastener cold heading, the spheroidization grade is generally required to be at least Grade 4 (out of 6, Grade 4 or above is good).

   • Reference: spheroidized carbides are uniformly distributed, no coarse lamellar pearlite.

3. Decarburization depth

   • If the protective atmosphere is poor during annealing, the surface may decarburize. Decarburization reduces the surface hardness of the finished bolt and can induce fatigue cracks.

   • Standards require decarburization depth not to exceed 1–2% of the thread height (depending on grade).

Real‑world case:

A batch of Grade 10.9 bolts exhibited thread “peeling” during assembly, and the customer complained of insufficient strength. Our inspection revealed that the raw material had a decarburization depth of 0.15 mm due to poor annealing atmosphere. After switching to QBH wire processed with controlled‑atmosphere spheroidizing annealing, decarburization was kept below 0.03 mm, and the problem was solved.

How do annealing, normalizing, quenching, and tempering differ? What comes after annealing?

Annealing is just one link in the fastener heat treatment chain. The table below clarifies the differences:

What happens after annealing:

• Spheroidized wire → pickling & phosphating (scale removal and lubrication) → cold heading → thread rolling → quenching + tempering → surface finishing.

• In short: Annealing paves the way for cold heading; quenching & tempering determine the final strength class.

How do annealing processes differ for different materials (carbon steel, alloy steel, stainless steel)?

Annealing temperatures and atmosphere requirements vary significantly by material. Key points for common fastener materials: