Answers to questions about heat treatment during steel rolling

1. What are the commonly used quenching methods, and explain the principles of choosing different quenching methods?

Quenching method:

1. Single-liquid quenching – the process of cooling to the end in a quenching medium, the single-liquid quenching structure stress and thermal stress are relatively large, and the quenching deformation is large.

2. Double-liquid quenching – purpose: rapid cooling between 650 ℃ ~ Ms, so that V>Vc, slow cooling below Ms, to reduce tissue stress. Carbon steel: water first, then oil. Alloy steel: oil first and then air.

3. Staged quenching – After the workpiece is taken out, it is kept at a certain temperature to make the temperature inside and outside the workpiece consistent, and then the process of air cooling. Staged quenching occurs when the M phase changes during air cooling, and the internal stress is small.

4. Austempering – refers to isothermal bainite transformation in the bainite temperature region, the internal stress is reduced, and the deformation is small. The principle of quenching method selection should not only consider meeting the performance requirements, but also reduce the quenching stress as much as possible to avoid quenching deformation and cracking.

II. What is the difference between chemical vapor deposition and physical vapor deposition technology, and their main applications

Chemical vapor deposition is mainly a CVD method. The reaction medium containing coating material elements is vaporized at a lower temperature, and then sent to a high-temperature reaction chamber to contact the surface of the workpiece to generate a high-temperature chemical reaction, and precipitate alloys or metals and their compounds to deposit on the workpiece. A coating is formed on the surface.

The main features of CVD method:

     1. Various crystalline or amorphous inorganic thin film materials can be deposited.

     2. High purity, strong collective binding force.

     3. The deposition layer is dense and has few pores.

     4. Good uniformity, simple equipment and process.

     5. The reaction temperature is higher.

Application: Preparation of films for various purposes on the surface of steel, cemented carbide, non-ferrous metals, inorganic non-metals and other materials, mainly insulator films, semiconductor films, conductor and superconductor films and corrosion-resistant films.

Physical vapor deposition: The process of depositing gaseous substances directly into a solid film on the surface of the workpiece is called PVD method. There are three basic methods, namely vacuum evaporation, sputter coating and ion plating. Application: wear-resistant coating, heat-resistant coating, corrosion-resistant coating, lubricating coating, functional coating decorative coating.

III. Explain the micro-morphology and macro-morphology of the fatigue fracture.

Micro: It is a strip pattern observed under a micro electron microscope, which is called fatigue strip or fatigue streak. Fatigue strips are ductile and brittle. Fatigue strips have a certain spacing, and each stripe corresponds to a stress cycle under certain conditions.

Macro: In most cases, it has the characteristics of brittle fracture, and no macroscopic deformation is visible to the naked eye. The area of the fatigue source is less and flat, sometimes it is a bright mirror surface, the crack extension area is like a river beach or a shell pattern, and there are some fatigue sources with different intervals as parallel arcs with the center of the circle. The microscopic morphology of the transient fracture zone depends on the characteristic load mode and size of the material, etc., which may be dimples or quasi-dissociation, dissociated intergranular fractures or mixed shapes.

IV. Point out three quality problems that often occur in induction heating quenching, and try to analyze the reasons.

1. Cracking: high heating temperature and uneven temperature; improper selection of quenching medium and temperature; untimely and insufficient tempering; high hardenability of materials, segregation of components, defects, and excessive inclusions; part design unreasonable.

2. Uneven surface hardness: unreasonable induction structure; uneven heating; uneven cooling; poor material organization (with banded structure, local decarburization.

3. Surface melting: the structure of the sensor is unreasonable; the parts have sharp corners, holes, bads, etc.; the heating time is too long, and there are cracks on the surface of the workpiece.