Key Questions and Answers on Heat Treatment in Steel Rolling Processes

Key Questions and Answers on Heat Treatment in Steel Rolling Processes

I. Common Quenching Methods and Selection Principles

Quenching Methods:

1. Single-liquid quenching – Cooling entirely in one quenching medium. This method generates significant structural and thermal stress, resulting in substantial deformation.

2. Double-liquid quenching – Aims to cool rapidly between 650°C and Ms temperature to achieve V > Vc, then cool slowly below Ms to reduce structural stress.

   • Carbon steel: water then oil

   • Alloy steel: oil then air

3. Interrupted (staged) quenching – The workpiece is held at a specific temperature to equalize internal and external temperatures, followed by air cooling. Martensitic transformation occurs during air cooling, minimizing internal stress.

4. Austempering – Involves isothermal transformation in the bainite temperature range, yielding low internal stress and minimal deformation.

Selection Principle: Choose methods that meet performance requirements while minimizing quenching stress to prevent deformation and cracking.

II. Chemical Vapor Deposition vs. Physical Vapor Deposition

Chemical Vapor Deposition (CVD):
A process where vaporized reaction media containing coating elements are introduced into a high-temperature chamber to react with the workpiece surface, depositing alloys, metals, or compounds.

CVD Characteristics:

1. Can deposit various crystalline/amorphous inorganic thin films

2. High purity, strong adhesion

3. Dense, low-porosity layers

4. Good uniformity, simple equipment/process

5. Requires high reaction temperatures

Application: Surface coatings on steel, cemented carbides, non-ferrous metals, and inorganic non-metals – including insulator, semiconductor, conductor, superconductor, and corrosion-resistant films.

Physical Vapor Deposition (PVD):
Direct deposition of gaseous substances onto workpiece surfaces as solid films. Three basic methods: vacuum evaporation, sputter coating, and ion plating.

Application: Wear-resistant, heat-resistant, corrosion-resistant, lubricating, functional, and decorative coatings.

III. Fatigue Fracture Morphology

Microscopic:
Appears as striations (fatigue streaks) under electron microscopy. Includes ductile and brittle types with characteristic spacing, where each striation corresponds to one stress cycle under specific conditions.

Macroscopic:
Typically exhibits brittle fracture characteristics with no visible deformation. Features include:

• Small, flat fatigue origin areas (sometimes mirror-like)

• Crack propagation zones resembling beach marks or shell patterns

• Multiple origins may show concentric arc patterns

• Final fracture zone morphology depends on material and load conditions (dimples, quasi-cleavage, intergranular fracture, or mixed modes)

IV. Common Induction Heating Quenching Defects and Causes

1. Cracking:

   • Overheating or uneven temperature

   • Improper quenchant selection/temperature

   • Delayed/inadequate tempering

   • High hardenability, composition segregation, defects, excessive inclusions

   • Poor part design

2. Uneven Surface Hardness:

   • Suboptimal inductor design

   • Non-uniform heating/cooling

   • Material issues (banded structure, localized decarburization)

3. Surface Melting:

   • Poor inductor configuration

   • Part geometry issues (sharp corners, holes, defects)

   • Excessive heating time or pre-existing surface cracks