110 Questions About Metal Heat Treatment (Part 3)

41. How does the metallurgical quality of steel affect the quenching crack?

Steel parts can be processed by forging, casting, cold-drawn steel, hot-rolled steel, etc. All kinds of blanks or materials may have metallurgical defects in the production process, or the metallurgical defects of raw materials may be left to the next process. Finally, these defects can expand into quenching cracks during quenching, or lead to the occurrence of cracks. For example, defects such as porosity, porosity, sand holes, segregation, and cracks may be formed inside or on the surface of steel casting due to improper processing technology in the hot working process; Shrinkage cavity, segregation, white spot, inclusion, crack, and so on Maybe formed in forging blank. These defects have a great influence on the quenching crack of steel. Generally speaking, the more serious the original defect, the greater the tendency of quenching crack.

42. What are the effects of carbon content and alloying elements on the cracking tendency of steel?

The carbon content and alloying elements of steel have important effects on the cracking tendency of steel. Generally speaking, with the increase of carbon content in martensite, the brittleness of martensite increases, the brittle fracture strength of steel decreases, and the quenching crack tendency increases. With the increase of carbon content, the influence of thermal stress is weakened and the influence of tissue stress is enhanced. When the workpiece is quenched in water, the surface compressive stress becomes smaller, and the tensile stress in the middle is close to the surface. When the oil is quenched, the surface tensile stress increases. All of these increase the tendency of quenching cracking. The influence of alloying elements on quenching is complex, and the thermal conductivity of steel decreases with the increase of alloying elements, which increases the phase transition heterogeneity. At the same time, with the increase of alloy content, the austenite is strengthened, and it is difficult to relax the stress by plastic deformation, thus increasing the internal stress of heat treatment and increasing the tendency of quenching. However, with the increase of the content of alloying elements, the hardenability of steel is improved. It can be quenched with a mild quenching medium, which can reduce the quenching tendency. In addition, some alloying elements such as vanadium, niobium, and titanium have the function of refining austenite grain, reducing the tendency of steel overheating, and thus reducing the tendency of quenching.

43. What is the effect of the original tissue on the cracking property?

The original microstructure of steel has a great influence on the crack before quenching. When the heating temperature of flake pearlite is too high, it is easy to cause the growth of austenite grain and easy to overheat. Therefore, the quenching heating temperature and holding time must be strictly controlled for the steel parts whose original structure is flake pearlite. Otherwise, it will cause quenching cracking due to the overheating of steel parts. Steel with spheroidal pearlite original organization, when quenching heating, globular carbide is stable because it is over, into austenite transformation process, the dissolution of carbide, often a small number of residual carbides, the residual carbides hindered the austenitic grain growth, compared with lamellar pearlite, quenching can obtain fine martensite, therefore the original organization for uniform spheroidal pearlite steel to reduce crack before quenching is the ideal state of the organization.

44. Why does the phenomenon of repeated quenching cracking occur?

In production, the phenomenon of repeated quenching cracking often occurs, which is caused by the direct secondary quenching without intermediate normalizing or intermediate annealing before the secondary quenching. There is no carbide in the structure that prevents the growth of austenite grains, so the austenite grains can easily grow significantly and cause overheating. Therefore, one intermediate annealing in the secondary quenching can also be used to completely eliminate the internal stress.

45.How the size and structure of the parts affect the cracking property?

The section size of the parts is too small and too much is not easy to crack. When the workpiece with a small section size is quenched, the heart is easy to be hardened, and the formation of martensite in the heart and the surface takes place almost at the same time, so the tissue stress is small and it is not easy to be quenched. Section size is too large parts, especially with low hardenability steel manufacturing, quenching not only the heart cannot harden, but even the surface also can not get martensite, the internal stress is mainly thermal stress, it is not easy to appear quenching crack. Therefore, for each kind of steel parts, under a certain quenching medium, there is a critical crack diameter, that is to say, in the critical diameter of the parts have a greater crack tendency. The size of the risk of cracking may vary depending on the chemical composition of the steel, the heating temperature, and the method used. The sharp Angle, angular Angle, and other geometric shape factors of the parts make the local cooling speed of the workpiece change sharply, increase the residual stress of quenching, and thus increase the cracking tendency of quenching. The increase of the non-uniformity of the section of the part, the quenching tendency is also increased, the thin part in the quenching martensite transformation occurs first, then, when the thick part of the martensite transformation, volume expansion, so that the thin part under tensile stress, stress concentration at the junction of the thin thickness, thus often appear quenching crack.

46. How do process factors affect quenching cracks?

Process factors (mainly quenching heating temperature, holding time, cooling mode, etc.) have a great influence on quenching crack tendency. Heat treatment includes the process of heating, holding, and cooling. Not only can cracks be generated during heat treatment (quenching), but they may also be formed during heating if not properly heated.

47.What cracks can be caused by improper heating?

Cracks caused by excessive heating rate, surface carbonization or decarburization, cracks caused by overheating or overburning, hydrogen-induced cracks caused by heating in a hydrogen-containing atmosphere.

48. Why does the excessive heating rate cause cracks?

Due to the different crystallization process of some materials in the casting process is bound to form non-metallic inclusions of non-uniform composition, non-uniform structure, and as-cast materials. Such as hard and brittle carbide phase in cast high manganese steel, composition segregation and porosity in cast high alloy steel and other defects, when the large workpiece is heated rapidly, the larger stress may be formed, thus cracking occurs.

49. Why does surface carburization or decarburization cause cracks?

When alloy steel parts are heated in a protective atmosphere furnace (or controlled atmosphere furnace) with hydrocarbon as gas source, due to improper operation or out of control, the carbon potential in the furnace increases, so that the surface carbon content of the heated workpiece exceeds the original carbon content of the workpiece. During the subsequent heat treatment, the operator still quenches the steel in accordance with the original process specifications, resulting in quenching cracks.

When the casting of high manganese steel is treated by heat treatment, if the surface layer is decarburized and demagnetized, cracks will appear on the workpiece surface. When low alloy tool steel and high-speed steel are heated in heat treatment, cracks may also occur if the surface is decarburized.

50. Why does overheating or overburning cause cracks?

High-speed steel, stainless steel workpiece, due to high quenching temperature, once the heating temperature is out of control, it is easy to cause overheat or overburn, thus causing heat treatment crack.

51.What kinds of pearlite are there? What are their morphological and functional characteristics?

The morphology of pearlite can be divided into two types: flake pearlite and granular pearlite.

Lamellar pearlite is composed of cementite and ferrite arranged alternately

(1) the formation of lamellar pearlite first on austenite grain boundary precipitation nucleation of cementite, and grew up in a sheet to appear on both sides of the lean carbon austenite, prompting ferrite on the austenite in the interface, the nucleation of cementite formation lamellar ferrite, and the nearby carbon-rich austenite prompted cementite along with the interface of austenite, ferrite nucleation. Such repeated alternation, eventually form the pearlite, when the above way to the horizontal development of pearlite at the same time, the flake ferrite front in the austenite cementite front diffusion, promote the Broadbent along with the longitudinal growth, resulting in the formation of the pearlite field. Within a single austenite grain, several pearlite domains may be formed.

(2) Lamellar spacing Pearlite lamellar spacing refers to the average distance between two adjacent cementitious in pearlite, the size of which mainly depends on the transition temperature (subcooling). The lower the transition temperature, the smaller the lamellar space, the finer the pearlite structure, and the greater the diffusion degree of cementite.B spheroidal pearlite, the formation of the spheroidal pearlite is also a process of cementite and ferrite alternate precipitation, among them, the precipitation of cementite is not within the austenitic grain soluble carbide fire carbon-rich OuDeFei spontaneous nucleation, due to the growth of approximate consistent, eventually in the ferritic matrix uniformly distributed on the granular (spherical cementite spheroidal pearlite, is thought to have lower austenitizing temperature is advantageous to the formation of granular pearlite.The mechanical properties of C pearlite and the strength and hardness of flake pearlite increase with the decrease of lamellar space. Granular pearlite has lower strength and hardness, better plasticity and toughness.

52. What measures can be taken to obtain a fine austenite grain size of steel during heating?

A: Heating temperature and holding time: The higher the temperature and the longer the holding time, the faster and larger the austenite grains grow. The growth rate of austenite grain increases exponentially with the increase of temperature. At high temperature, the effect of holding time on grain growth is greater at low temperature.

B: Heating rate: The higher the heating rate and the higher the superheat, the higher the actual temperature of austenite formation, because the ratio of nucleation rate and growth rate increases. Thus, small initial grains can be obtained. This also indicates that rapid heating can produce fine austenite grains.

C: Chemical composition of steel: Austenite grains tend to grow and coarseness with the increase of carbon content of steel, but not enough to form undissolved carbide. Thus, eutectoid carbon steel is more sensitive to overheating than hypereutectoid carbon steel.

D: The original structure of steel: generally, the finer the original structure is or the non-equilibrium structure is, the greater the carbide decomposition degree is, the smaller the austenite initial grain is obtained, but the grain growth tendency of steel increases, and the overheating sensitivity increases. Therefore, it is not suitable to use too high heating temperature and too long holding time for the steel with a very fine original structure.

53. How does the temper brittleness of the first and second classes occur? How do you get rid of temper brittleness?

Class I temper brittleness (temper Martensite brittleness): Carbon steel will temper in the temperature range of 200~400°C, the impact toughness will decrease at room temperature, resulting in brittleness, namely class I temper brittleness or temper Martensite brittleness. Brittleness of alloy steels occurs in a slightly higher temperature range, about 250~450 degrees.

If the first type of temper brittleness occurs after the part is tempered, it needs to be reheated and quenched to eliminate it.

The second type of tempering brittleness (martensite high-temperature tempering brittleness or reversible tempering brittleness): The impact toughness of some alloy steels decreases when they are cooled slowly after tempering within the temperature range of 450~650 degrees. If the resulting brittle steel is reheated to a predetermined tempering temperature (slightly above the temperature range that causes embrittlement) and then rapidly cooled to room temperature, brittleness will disappear. For this reason, also known as reversible temper brittleness.

54.What is the hardenability of steel? What factors affect hardenability?

A: The ability of steel to obtain martensite at quenching, that is, the depth at which the steel is quenched, is called hardenability. The hardenability of steel depends on its critical cooling rate. The more right the C curve is, the smaller the critical cooling speed and the greater the hardenability.

B: 1. Influence of carbon content: With the increase of carbon content in austenite, the stability increases, making the C curve move to the right.

2. Influence of alloying elements: alloying elements (except Co) can improve the hardenability of steel.

3. Influence of austenitizing temperature and holding time: the higher the austenitizing temperature, the longer the holding time, the more complete the carbide dissolution, the larger the austenitic grain, the smaller the total boundary area, and the smaller the nucleation, thus delaying the pearlite transformation by the right shift of C curve. In a word, the faster the heating rate, the shorter the holding time, the smaller the austenite grain, the more heterogeneous the composition, and the more undissolved second phase, the faster the isothermal transformation speed, making the C curve move to the left.

55. The austenite grain growth should be controlled during heat treatment. The factors affecting the austenite grain growth and the measures to control the austenite grain growth should be analyzed.

Heating temperature and holding time: The higher the heating temperature, the longer the holding time, and the larger the austenite grains, the more important the heating temperature is.

Heating speed: the faster the heating speed is, the higher the superheat is, the higher the ratio of nucleation rate and growth speed is to refine the grains, and the higher the actual grain size of austenite is. Chemical composition of steel:

1. Carbon steel – eutectoid steel is easier to overheat than hypereutectoid steel;

2. Alloy steel — Carbon and nitrous compounds, such as Ti, V, Vr, Nb, W, Mo, Cr, etc. are added into the steel to form elements, which strongly hinder the migration of austenite grain boundary and make the grain refined. The steel deoxidized with Al has a fine grain, while the steel deoxidized with Si has coarse grain.

The original structure – The finer the original structure or the non-equilibrium structure, the larger the grain size tendency of the steel and the easier the grain coarsening.

56.How many types of cast iron are usually divided into?

The forms of carbon in these cast iron and their effects on cast iron properties are indicated respectively.

Gray cast iron: high compressive strength, excellent wear resistance, and vibration suppression, low notch sensitivity.

Ductile iron: both gray cast iron and medium carbon steel tensile strength, bending fatigue strength, and good shape and toughness.

Graphite of malleable cast iron is flocculent and has a little cutting effect on the matrix, so its strength, plasticity, and toughness are higher than gray cast iron, especially pearlite malleable iron can be comparable to cast steel, but it cannot be forged.

Vermicular cast iron: The tensile strength, plasticity and fatigue strength of vermicular cast iron are better than gray cast iron, and ductile cast iron is close to the ferrite matrix. In addition, its thermal conductivity, casting, machinability are better than ductile iron, and gray cast iron similar.

Give examples and briefly explain which effective heat treatment techniques can be used to improve the die life. Please give more than five examples.

The known processing route of GCr15 steel precision bearing is as follows:

Blanking – forging – superfine treatment – machining – quenching – cold treatment – stabilization treatment. The heat treatment process includes:

Ultra-fine heat treatment process is 1050℃×20 ~ 30min high-temperature heating, 250 ~ 350℃×2h salt bath isothermal, 690 ~ 720℃×3h with furnace cooling to 500℃ air cooling.

Quenching: heating at 835 ~ 850℃×45 ~ 60min in a protective atmosphere, cooling in oil at 150 ~ 170℃ for 5 ~ 10min, then cooling in oil at 30 ~ 60℃.

Cold treatment: cold treatment at -40 — -70℃×1 ~ 1.5h after cleaning

Stabilization heat treatment: 140 ~ 180℃×4 ~ 12h after rough grinding;After fine grinding, 120 ~ 160℃×6 ~ 24h.

57.Why the material of machine gear is usually 45 steel, while the material of automobile gear is 20CrMnTi, etc. Please formulate the process route and the purpose of adopting the heat treatment process.

(1) Machine tool gears work smoothly without strong impact, the load is not big, the speed is medium, on the gear core strength and toughness requirements are not high, generally choose 40 or 45 steel manufacturing. Working condition of automobile and tractor gear than bad machine gear, more stress, overload and hit frequently, while starting, braking and speed on the abrasion resistance, bending fatigue strength, contact fatigue strength, core strength and toughness of performance requirements are relatively high, with medium carbon steel or carbon in the low alloy by high frequency induction heating surface quenching can not guarantee performance.

(2) Machine tool gear processing process: blanking — forging — normalizing — tempering — semi-finishing — high-frequency induction heating surface quenching + low temperature tempering — fine grinding — finished products. Normalizing can homogenize the structure, eliminate the forging stress, and adjust the hardness to improve the machinability. The quenching and tempering treatment can make the gear have higher comprehensive mechanical properties, improve the strength and toughness of the tooth core, make the gear can withstand greater bending stress and impact load, and reduce the quenching deformation. High-frequency induction heating surface quenching can improve the gear surface hardness and wear resistance, improve the tooth surface contact fatigue;Low temperature tempering eliminates quenching stress without reducing surface hardness. Prevent grinding cracks and improve the impact resistance of gear.

Automobile gear processing technology route: blanking – forging – normalizing – machining – carburizing, quenching + low-temperature tempering – shot peening – grinding – finished product. Normalizing treatment can make the structure even and adjust the hardness to improve the machinability. Carburizing is to improve the mass fraction of tooth surface carbon (0.8-1.05%); Quenching can improve the hardness of tooth surface and obtain a certain depth of hardened layer (2.8-1.3mm), improve the wear resistance and contact fatigue strength of tooth surface; The function of low temperature tempering is to eliminate quenching stress, prevent grinding crack and improve impact resistance. Shot-peening treatment can improve the hardness of the tooth surface by about 1-3HRC, increase the residual compressive stress on the surface, and thus improve the contact fatigue strength.

58. Types and solutions of temper brittleness

Tempering brittleness: the phenomenon that the impact toughness and brittleness of quenched steel decrease and increase obviously with the increase of tempering temperature in a certain tempering temperature range. There are two categories, the first and the second.

Type I: irreversible tempering embrittlement of hardened steel during tempering of 250~400; Type 2:450~650 reversible.

Methods: The first type of production can not be eliminated, you can add SI, make the brittle transition temperature rise to more than 300, and then temper at 250; The second type: in the brittle temperature short time tempering, fast cooling does not occur, slow cooling. Reheating short – time temper at brittle temperature, quick cooling can be eliminated.

59. Purpose of micro-thinning heat treatment of cold-working die steel?Cyclic superfine treatment of Cr12MoV steel?

Objective: Microrefining heat treatment includes the refinement of the steel matrix and the refinement of carbide. The microstructure refinement can improve the strength and toughness of steel, and the carbide refinement can improve the strength, toughness, and wear resistance of steel.

Process: 1150 heating quenching +650 tempering +1000 heating oil quenching +650 tempering +1030 heating oil quenching 170 isothermal 30min air cooling +170 tempering.

How many kinds of martensite are common in quenched steels? The structure? Performance features? Forming conditions?

Slats and flaps. The substructure of slat is dislocation, with high strength and hardness, good plasticity and toughness. Forming conditions of low carbon steel, above 200℃ temperature. The lamellae with medium and high carbon content below 200℃ are twin crystals with high hardness and brittleness.

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