14 Questions on basic knowledge of material heat treatment

1. What are the commonly used quenching methods? Explain the principles for choosing different quenching methods?

(1) Single liquid quenching - a process of cooling to the bottom in a quenching medium. The structural stress and thermal stress of single liquid quenching are relatively large, and the quenching deformation is large.

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

(3) Graded quenching - a process in which the workpiece is taken out and held at a certain temperature to make the internal and external temperatures of the workpiece consistent, and then air-cooled. Graded quenching is a process in which M phase change occurs during air cooling, and the internal stress is small.

(4) Isothermal quenching - refers to isothermal in the bainite temperature zone, bainite transformation occurs, the internal stress is reduced, and the deformation is small.

The principle of selecting the quenching method must not only meet the performance requirements, but also reduce the quenching stress as much as possible to avoid quenching deformation and cracking.

2. Briefly describe the quenching and cooling method.

1) Water cooling: used for carbon steel workpieces with simple shapes, mainly quenched and tempered parts;

2) Oil cooling: alloy steel, alloy tool steel workpieces.

3) Delayed quenching: The workpiece is cooled in the air before being immersed in the coolant to reduce thermal stress;

4) Dual medium quenching: The workpiece is generally immersed in water for cooling first, until it is cooled to near the point where martensite begins to transform, and then immediately transferred to oil for slow cooling;

3. Briefly describe the main features of commonly used chemical heat treatments (carburizing, nitriding, carbonitriding and nitrocarburizing), the microstructure and performance characteristics after heat treatment, and what materials or parts are they mainly suitable for?

Carburizing: It is mainly the process of infiltrating carbon atoms into the surface of the workpiece. The surface layer is tempered with martensite, residual A and carbides. The purpose of the core part is to increase the carbon content of the surface layer, which has high hardness and high wear resistance. The core part has a certain strength. And high toughness, allowing it to withstand large impacts and friction, low carbon steel such as 20CrMnTi is commonly used in gears and piston pins.

Nitriding: Nitrogen atoms are infiltrated into the surface to improve the surface hardness, wear resistance, fatigue strength, corrosion resistance and thermal hardness. The surface layer is nitride and the core is tempered sorbite. Gas nitriding, liquid nitriding, etc. are commonly used. 38CrMoAlA, 18CrNiW.

Carbonitriding: Carbonitriding has low temperature, fast speed, and small deformation of parts. The surface structure is fine needle tempered martensite + granular carbonitride Fe3 (C, N) + a small amount of retained austenite. It has high wear resistance, fatigue strength and compressive strength, as well as certain corrosion resistance. Often used in heavy and medium load gears made of low and medium carbon alloy steel.

Nitrocarburizing: The nitrocarburizing process has a faster co-carburizing speed, and the surface hardness is slightly lower than that of nitriding, but it has good fatigue resistance. It is mainly used for parts and tools that are subject to small impact loads and require wear resistance, high fatigue limit and small deformation. General steel parts: carbon structural steel, alloy structural steel, alloy tool steel, gray cast iron, ductile iron and powder metallurgy can all be nitrocarburized.

4. Briefly describe the principles of heat treatment process design

1) Advanced technology

2) The process is reliable, reasonable and feasible

3) Economy of process

4) Safety of the process

5) Try to use mechanized and highly automated process equipment

5. What issues should be considered in the optimized design of the heat treatment process?

1) Fully consider the connection between hot and cold processing techniques, and the arrangement of heat treatment procedures must be reasonable;

2) Use new technologies as much as possible, briefly describe the heat treatment process, and shorten the production cycle. Under the condition of ensuring the required organization and performance of the parts, try to combine different processes or processes with each other;

3) Sometimes in order to improve product quality and extend the service life of workpieces, it is necessary to add a heat treatment process.

6. Precautions for tempering

Tempering must be done in time, and parts should be tempered within 4 hours after quenching. Commonly used tempering methods include self-tempering, furnace tempering and induction tempering.

7. What are the commonly used cooling media?

Water, salt water, alkaline water, mechanical oil, nitrate salt, polyvinyl alcohol, trinitrate aqueous solution, water-soluble quenching agent, special quenching oil, etc.

8. What measures should be taken to obtain fine austenite grains when steel is heated?

Measures: lower the temperature, reduce the holding time, increase the heating speed, and add strong carbon and nitride-forming elements to the steel.

9. What are the main measures and process methods to reduce heat treatment distortion of parts?

Reduce stress concentration; slow down heating and cooling rates; place parts reasonably; choose appropriate tooling.

10. Try to point out three defects that often occur after heat treatment of carburized parts, and analyze their causes and preventive measures.

1) The hardness after quenching is low: mainly due to the low carbon concentration in the deep surface layer or surface decarburization; the quenching process is unreasonable, there is no quenching or there is too much residual austenite.

2) The depth of the carburizing layer is not enough: the main reason is that the furnace temperature is low, the time is short, or the atmosphere circulation in the furnace is poor, the surface of the part is not clean, and the carbon potential is too high, and there is carbon on the surface of the workpiece; increase the temperature and time of carburizing, and clean the surface of the workpiece before loading into the furnace. , reasonably control the carbon potential.

3) Large pieces of Wangzhuang carbide appear in the carburizing layer; the main reason is that the surface carbon concentration is too high during carburizing, which reduces the activity of the penetrating agent and strictly controls the carbon potential.

11. In order to improve the cutting performance, what kind of heat treatment should be carried out for 15Cr, 20Cr2Ni4, 40Cr and 5CrMnMo?

15Cr: normalizing;

20Cr2Ni4: normalizing + tempering;

40Cr: tempering;

5CrMnMo: annealing;

12. The hardness of 45 steel is required to be 217HB~255HB after quenching and tempering.

Q: The hardness is found to be too high after heat treatment. Can the hardness be reduced by slowing down the cooling rate during tempering?

A: No, the tempering temperature needs to be adjusted;

Q: The hardness is low after heat treatment, can the hardness be increased by lowering the temperature during tempering?

A: No, you need to lower the tempering temperature after re-quenching.

97. Please indicate the temperature range for tempering after overall quenching of the following workpieces.

(1) Molds, bearings, gears: low temperature: 150-250℃;

(2) Spring: medium temperature: 350-500℃;

(3) Shafts, connecting rods, bolts: high temperature: 500-650℃.

13. Try to point out 5 types of commonly used electric heating element materials, and explain their applicable temperature ranges.

Iron-chromium-aluminum electric heating alloy 1Cr13Al4, used in low temperature 0Cr25Al5, 0Cr13Al16Mo2, suitable for medium temperature 0Cr27AlMo2, used in high temperature up to 1300; nickel-chromium electric heating alloy Cr15Ni60, used in low temperature. Cr20Ni80, used in medium temperatures below 1000 degrees; SiC electric heating elements can work for a long time at 1350 degrees, with a maximum temperature of 1500; MoSi2 electric heating elements, with a maximum temperature of 1700; graphite electric heating elements, electric heating elements of vacuum furnaces, with an operating temperature of 1400-2800℃.

14. What types of commonly used controlled atmospheres are there? Briefly describe the characteristics and applications of each atmosphere.

There are endothermic atmospheres, dripping atmospheres, straight atmospheres, and other controllable atmospheres (nitrogen atmosphere, ammonia decomposition atmosphere, exothermic atmosphere), etc.

1) Endothermic atmosphere is to mix the raw material gas with air in a certain proportion, and pass through the catalyst at high temperature to react to generate an atmosphere mainly containing CO, H2, N2 and trace amounts of CO2, O2 and H2O. Since the reaction needs to absorb heat, It's called endothermic atmosphere or RX gas. Used for carburizing and carbonitriding.

2) The dripping atmosphere is to directly point methanol into the furnace for cracking to generate a carrier containing CO and H2, and then add enrichment agents for carburizing; carbonitriding at lower temperatures, protected heating and bright quenching, etc.

3) Mix the carburizing agent such as natural gas and air in a certain proportion and then directly pass it into the furnace. The carburizing atmosphere will be directly generated by the 900°C reaction at high temperature. Ammonia decomposition gas is used as nitriding carrier gas and low-temperature heating protective atmosphere for steel or non-ferrous metals. Nitrogen-based atmosphere has a good protective effect on high carbon steel or bearing steel. Exothermic atmospheres are used for bright heat treatment of mild steel, copper or decarburization annealing of malleable iron.