Views: 0 Author: Site Editor Publish Time: 2023-08-25 Origin: Site
This is an article one after 30 Questions About Heat Treatment (2) that answers another 10 asked questions about heat treatment, here is the catalogue of these 10 questions.
23. What are the main ways to accelerate chemical heat treatment?
24. What are the three basic heat transfer methods?
25. What is the black tissue that appears in carbonitriding? How to prevent it from happening?
26. Briefly describe the selection principle of induction heating and quenching process parameters.
27. What are the precautions for tempering?
28. Adjustment of electric parameters of induction heating.
29. What are the commonly used cooling media?
30. Try to analyze the factors affecting the hardenability of steel?
21.What is Fracture Toughness? How to judge whether a part has low stress brittle fracture according to the fracture toughness K1C of the material, the working stress σ of the part and the crack radius length α in the part?
The performance index that indicates the ability of a material to resist fracture is the fracture toughness. If K1>K1C, the material has low stress brittle fracture.
Compared with the steel phase transformation characteristics of gray cast iron:
1. Cast iron is a Fe-C-Si ternary alloy, and the eutectoid transformation is in a wide temperature range, and there are ferrite + austenite + graphite at this temperature;
2. The graphitization process of cast iron is easy to carry out, and the process can be controlled to obtain cast iron with ferrite matrix, pearlite matrix and ferrite + pearlite matrix;
3. By controlling the austenitizing temperature heating, heat preservation and cooling conditions, the carbon content of A and transformation products can be adjusted and controlled within a considerable range;
4. Compared with steel, the diffusion distance of carbon atoms is longer;
5. The heat treatment of cast iron cannot change the shape and distribution of graphite, but can only change the collective structure and properties.
22.What is the basic process of A formation when steel is heated? Factors affecting the grain size of A?
Formation process: the formation of A crystal nuclei, the growth of A grains, the dissolution of residual cementite, and the homogenization of A; factors: heating temperature, holding time, heating speed, steel composition, and original structure.
23.What are the main ways to accelerate chemical heat treatment?
Approaches: segmental control method, compound infiltration treatment, high temperature diffusion, new materials that accelerate the diffusion process, chemical infiltration and physical infiltration.
24.What are the three basic heat transfer methods?
Heat transfer mode: conduction heat transfer, convective heat transfer, radiation heat transfer (radiation heat transfer for vacuum furnaces above 700°C).
25.What is the black structure that appears in carbonitriding? How to prevent it from happening?
Black structure refers to black spots, black belts, and black nets. In order to prevent the appearance of black tissue, the nitrogen content in the infiltrated layer should not be high enough, generally more than 0.5%, it is easy to appear black spots; the nitrogen content in the infiltrated layer should not be too low, otherwise it is easy to form a Troostite network. In order to suppress the Troostite network, the amount of ammonia added should be moderate. If the amount of ammonia gas is too high, the dew point of the furnace gas will decrease, which will promote the appearance of black tissue.
In order to suppress the appearance of the Troostite network, the quenching heating temperature can also be appropriately increased or a cooling medium with a strong cooling capacity can be used. When the depth of black tissue is less than 0.02mm, it is remedied by shot peening.
26.Briefly describe the selection principle of induction heating quenching process parameters.
Heating method: Induction heating quenching has two methods: simultaneous heating once quenching and mobile heating continuous quenching, depending on equipment conditions and parts types. At the same time, the specific power of heating is generally 0.5~4.0kw/㎠, and the specific power of mobile heating is generally greater than 1.5kw/㎠. Continuous quenching is used for long shaft parts, tubular inner hole quenched parts, medium modulus gears with wide tooth width, and strip-shaped parts; single-tooth continuous quenching is used for extra large gears.
Heating parameters:
1. Heating temperature, due to the fast induction heating, in order to fully transform the structure, the quenching temperature is 30-50°C higher than that of general heat treatment;
2. Heating time: It depends on various factors such as the technical requirements of the part, material, shape, size, current frequency, specific power, etc.
Quenching cooling method and quenching medium: The quenching cooling method of quenching heating usually adopts spray cooling and intrusion cooling.
27.What are the precautions for tempering?
Tempering must be timely, and the parts should be tempered within 4 hours after quenching. Commonly used tempering methods include self-tempering, furnace tempering and induction tempering.
28.Adjustment of induction heating electrical parameters.
The purpose is to make the high and medium frequency power supply work in a resonant state, so that the equipment can play a higher efficiency.
1. Adjustment of high-frequency heating electric parameters. Under the low voltage load condition of 7-8kv, adjust the coupling, feedback the position of the handwheel to make the ratio of grid current to anode current 1:5-1:10, and then raise the anode voltage to the working voltage to further adjust the electrical parameters. Adjust the tank circuit voltage to the desired value for the best match.
2. Adjust the electric parameters of the intermediate frequency heating, select the appropriate turns ratio and appropriate capacitance of the quenching transformer according to the size of the part, the length of the shape hardening area and the structure of the inductor, so that it can work in a resonance state.
29.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.
30.Try to analyze the factors affecting the hardenability of steel?
1. The influence of carbon content: the stability of hypoeutectoid steel increases with the increase of carbon content, and the C curve shifts to the right; the hypereutectoid steel increases with the increase of carbon content, the increase of unmelted carbides, and the stability of A Decrease, the C curve shifts to the right.
2. The influence of alloying elements: Except for Co, all metal elements in solid solution state shift the C curve to the right.
3. A curing temperature and holding time: the higher the A curing temperature, the longer the holding time, the more complete the dissolution of carbides, the coarser the A grains, and the right shift of the C curve.
4. The influence of the original tissue: the thinner the original tissue, the easier it is to obtain a uniform A, which makes the C curve move to the right and Ms to move down.
5. The influence of stress and strain makes the C curve shift to the left.
