110 Questions About Metal Heat Treatment (Part 2)

21. What is fracture toughness? How to determine whether a part has low stress brittle fracture according to the fracture toughness of K1c, the working stress of the part, and the crack radius of the part?

Fracture toughness is the property index that indicates the ability of the material to resist fracture.According to the formula, = s/k K1= √ K1C= √ if K1>At K1c, low stress brittle fracture occurs

1. Phase transition characteristics of gray cast iron compared with steel :(1) cast iron is a fe-c-si ternary alloy, eutectoid transformation in a wide temperature range, where there is ferrite + austenite + graphite;2) The graphitization process of cast iron is easy to be carried out. By controlling this process, ferrite matrix, pearlite matrix, and ferrite + pearlite matrix of cast iron are obtained;(3) The carbon content of A and the transformed products can be adjusted and controlled in A considerable range by controlling the austenitizing temperature, heat preservation and cooling conditions;(4) Compared with steel, carbon atoms have a longer diffusion distance;(5) The heat treatment of cast iron cannot change the shape and distribution of graphite, but only the collective structure and performance.

22. The basic process of A formation when steel is heated? What factors affect the grain size of A?

Formation process: the formation of A crystal nucleus, the growth of A grains, the dissolution of residual cementite, the homogenization of A; Factors: heating temperature, holding time, heating speed, steel composition, original structure.

23.What are the main ways to accelerate chemical heat treatment?

Compare the characteristics of the first stage carburizing, second stage carburizing, and dynamic carbon potential control.

Approaches: subsection control method, composite seepage treatment, high temperature diffusion, the use of new materials to accelerate the diffusion process, chemical penetration, physical penetration.

24. What are the three basic ways of heat transfer?

The application of energy saving in the heat treatment furnace is illustrated respectively.

Heat transfer mode: conduction heat transfer, convection heat transfer, radiation heat transfer; Not found (above 700 ° C vacuum furnace is radiant heat transfer)

25. What is the black tissue in carbonitriding? How can they be prevented?

Black tissue refers to black spots, black belts, and black nets. In order to prevent the appearance of black tissue, the nitrogen content in the permeation layer should not be high enough, generally, more than 0.5% is easy to appear dot black tissue;The nitrogen content in the permeation layer should not be too low, otherwise, it is easy to form martensite network. In order to inhibit the torstenite network, the addition amount of ammonia should be moderate, while the ammonia content is too high and the furnace air dew point decreases, which will promote the appearance of black tissue.

In order to suppress the appearance of torstenite net, the quenching temperature can be properly increased or the cooling medium with strong cooling capacity can be used. When the depth of black tissue is less than 0.02mm, spray finishing reinforcement is adopted.

26. The selection principle of induction heating quenching process parameters is briefly described

Heating method: Induction heating quenching has two methods: one is simultaneous heating and the other is mobile heating and continuous quenching. The specific power of simultaneous heating is generally 0.5~4.0 kW/cm2, and the specific power of mobile heating is generally greater than 1.5 kW/cm2. Long shaft parts, tubular inner hole quenched parts, medium modulus gears with wide teeth and strip parts shall be quenched continuously; Large gear adopts single tooth continuous quenching.

Heating parameters: 1. Heating temperature. Due to the fast induction heating speed, the quenching temperature is 30-50 degrees higher than the general heat treatment in order to make the microstructure fully transformed; 2.2. Heating time: depending on the technical requirements, material, shape, size, current frequency, specific power, and other factors.

Quenching cooling method and quenching medium: The quenching cooling method of quenching heating usually adopts injection cooling and intrusion cooling.

27. Precautions for tempering

Tempering must be done in a timely manner, and the parts shall be tempered within 4h after quenching. The common methods of tempering are self-tempering, in-furnace tempering and induction tempering.

28. Adjust the electric parameters of induction heating

The purpose is to make the work of high and medium frequency power in the resonant state so that the equipment plays a higher efficiency.1. Adjustment of high-frequency heating electric parameters (under the condition of low voltage load of 7-8kV, adjust the coupling and feedback the position of handwheel to make the ratio of grid current and anode current 1:5-1:10, and then raise the anode voltage to the service voltage to further adjust the electric parameters to make the slot voltage adjust to the required value and achieve the best matching).2. Adjust the electric parameters of intermediate frequency heating. Select the proper turn ratio and capacitance of quenched transformer according to the size of parts, the length of hardening area in shape, and the structure of the inductor, so that it can work under resonance state.

29. What are the common cooling media?

Water, brine, caustic water, mechanical oil, saltpeter, polyvinyl alcohol, water-soluble quench, special quench oil, etc.

30. Try to analyze the factors affecting the hardenability of steel.

Influence of carbon content: with the increase of carbon content, the stability of A increases, and the C curve shifts to the right; With the increase of carbon content in hypereutectoid steel and the increase of unmelted carbide, the stability of A decreases, and the influence of C curve right shift of alloying elements: except Co, the solid solution state metal elements all move right of C curve of A transformation temperature and holding time.The higher the Temperature of A, the longer the holding time, the more complete the carbide dissolution, the larger the grain size of A, C curve right shift the effect of the original structure; The finer the original structure is, the easier it will be to get uniform A, which will cause the C curve to move to the right and the stress strain of Ms to move down. I’m going to shift C to the left.

31. What is the tissue and purpose of low temperature tempering?

Low tempering temperature (150-250 degrees)

The result of low temperature tempering is tempered martensite. Its purpose is to reduce the quenching stress and brittleness of the hardened steel under the premise of maintaining its high hardness and high wear resistance, so as to avoid cracking or premature damage during use. It is mainly used for all kinds of high carbon cutting tools, measuring tools, cold stamping dies, rolling bearings and carburizing parts, etc. The hardness after tempering is generally HRC58-64.

32. What is the organization and purpose of moderate tempering?

Moderate tempering (350-500 degrees)

The tissue obtained from moderate tempering is a tempered troostite. The aim is to obtain high yield strength, elastic limit, and high toughness. Therefore, it is mainly used for the treatment of a variety of springs and hot die, after tempering hardness is generally HRC35-50.

33. What is the tissue obtained from high temperature tempering and what is its purpose?

High tempering temperature (500-650 degrees)

The microstructure obtained from high temperature tempering is tempered Soxhlet. Traditionally, the heat treatment combining quenching and tempering at high temperature is called tempering treatment. Its purpose is to obtain the comprehensive mechanical properties of strength, hardness, plasticity, and toughness. Therefore, widely used in cars, tractors, machine tools, and other important structural parts, such as connecting rods, bolts, gears, and shafts. After tempering, the hardness is generally HB200-330.

34.What is normalizing?

Refers to the heat treatment process of heating steel or steel parts to or above (the upper critical point temperature of steel) and cooling them in still air after being kept at an appropriate time of 30 ~ 50℃.

35. What is the purpose of normalizing?

It is mainly to improve the mechanical properties of low carbon steel, improve machinability, refine grain, eliminate microstructure defects, and prepare for subsequent heat treatment.

The main purpose of normalizing medium and low carbon steel castings and forgings is to refine microstructure. Compared with annealing, the pearlite laminates after normalizing are thinner and ferrite grains are smaller, so the strength and hardness are higher.

Because the hardness of low carbon steel after annealing is too low, the phenomenon of sticking the knife occurs during cutting, and the cutting performance is poor. By increasing the hardness by normalizing, the cutting performance can be improved. Some parts of medium carbon structural steel can be normalized instead of tempered to simplify the heat treatment process.

Hypereutectoid steel normalizing heating knife Acm, make the original network of cementite all dissolve into austenite, and then use a faster rate of cooling, to inhibit the precipitation of cementite in the austenite grain boundary, so as to eliminate the network of carbide, improve the structure of hypereutectoid steel.

For parts requiring weld strength, normalizing is used to improve weld structure and ensure weld strength.

In the process of heat treatment, the repaired parts must be normalized, and the structural parts requiring mechanical properties must be tempered after normalization to meet the mechanical properties requirements.After normalizing medium and high alloy steel and large forgings, high temperature tempering must be added to eliminate the internal stress during normalizing.

36. What is quenching?

It refers to the heat treatment process of obtaining martensite (or bainite) tissue by heating the steel to a temperature above Ac3 or Ac1 (the lower critical point temperature of steel) for a certain period of time and then at an appropriate cooling rate. Common quenching processes include salt bath quenching, martensite graded quenching, bainite isothermal quenching, surface quenching, and local quenching, etc.

37. What is the purpose of quenching?

The purpose of quenching is to make the supercooled austenite undergo martensite or bainite transformation to obtain martensite or bainite structure, and then with tempering at different temperatures to greatly improve the strength, hardness, wear resistance, fatigue strength, and toughness of steel, so as to meet the different use requirements of various mechanical parts and tools. It can also satisfy the special physical and chemical properties such as ferromagnetism and corrosion resistance of some special steels through quenching.

38. How to determine the heating and holding time?

In actual production, the choice of heating temperature should be adjusted according to the specific situation. If the carbon content of subeutectoid steel is the lower limit, the upper limit of temperature can be selected when more furnace is installed and the depth of the hardening layer of the parts is to be increased. If the workpiece shape is complex, deformation requirements are strict, such as the use of temperature lower limit.

The duration of heat preservation is determined by the heating mode of the equipment, the size of the parts, the composition of steel, the amount of furnace, and the power of the equipment. For the whole quenching, the purpose of heat preservation is to make the workpiece internal temperature uniform tend to be the same. For all kinds of quenching, the holding time ultimately depends on getting a good quenching heating structure in the area requiring quenching.

Heating and heat preservation are important factors affecting the quenching quality, and the microstructure obtained by austenitizing directly affects the quenching performance. Generally, austenite grain of steel parts is controlled at 5 ~ 8 grades.

39. How to control the cooling rate?

The cooling rate must be higher than the critical cooling rate of steel in order to transform the medium temperature austenite into the low temperature metastable phase martensite during the cooling process. During the cooling process of the workpiece, there is a certain difference between the cooling speed of the surface and the core. If the difference is large enough, the part larger than the critical cooling speed may be converted into martensite, while the part smaller than the critical cooling speed cannot be converted into martensite. In order to ensure the transformation of the whole section into martensite, the quenching medium with sufficient cooling capacity should be selected to ensure that the workpiece center has sufficient cooling speed. But the cooling rate is large, the workpiece interior due to heat expansion and cold shrinkage uneven internal stress, may make the workpiece deformation or crack. Therefore, the two contradictory factors mentioned above should be considered and the quenching medium and cooling mode should be selected reasonably.

In the cooling stage, it is the key link of the quenching process not only to get the reasonable structure of the parts and achieve the required performance but also to keep the dimension and shape precision of the parts.

40. What are the influencing factors of quenching crack?

There are many factors that affect the formation of quenching cracks in steel parts, including metallurgical, structural, and technological factors. It is of great significance to understand the effects of various factors on quenching cracks and to prevent the occurrence of quenching cracks and improve the yield.

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