High-speed steel is a blue-brown steel, contains a large number of alloying elements, after smelting a large number of primary eutectic carbides and secondary carbides (about 18% to 22% of the total composition), which quench the quality of high-speed steel cutting tools and The service life has a great influence. High-speed steel quenching temperature is close to the melting point, there is still 25% to 35% of retained austenite in the quenched structure, resulting in high-speed steel tools are prone to cracks and corrosion. The following analyzes the causes of quenching cracks and corrosion of HSS cutters and proposes corresponding preventive measures. 1 Metallurgical Defects of High Speed ​​Steel Raw Materials A large number of carbides contained in high speed steel are hard and brittle and are brittle. A primary eutectic carbide is distributed in the steel matrix in the form of a coarse skeleton (or dendritic). After the billets are rolled and rolled by the bloom, although the alloy carbides are broken and refined to a certain degree, the segregation of the carbides still exists, and is distributed along the rolling direction in the form of band, full network, semi-meshed, or stacked. . The degree of carbide unevenness increases as the raw material diameter or thickness increases. The eutectic carbides are quite stable, and conventional heat treatment is difficult to eliminate, which can lead to stress concentration and become a source of quench cracks. The segregation or excess of impurities such as sulfur and phosphorus in the steel is also an important cause of quench cracking. High-speed steel has poor thermal conductivity and thermoplastic properties, and has large deformation resistance. When hot working, it easily leads to the formation of micro-cracks in the surface and inner layers of the metal. Eventually, the material is scrapped due to crack propagation during quenching. Macroscopic metallurgical defects such as porosity, shrinkage pores, bubbles, segregation, white spots, dendritic crystallization, coarse grains, inclusions, internal cracks, hairline, large-grain carbonization of large-scale steel ingots during smelting, rolling, or forging, etc. Materials and non-metal slag inclusions, etc., can easily lead to stress concentration during quenching, and quench cracks can occur when the stress is greater than the material strength limit. The preventive measures are as follows: 1Select small steel ingots to start rolling a variety of cutting tool raw materials; 2Select secondary refining electroslag remelting steel ingots, which have high purity, less impurities, fine grains, small carbides, uniform structure, no The advantages of macro-metallurgical defects; 3 Forging of unqualified raw materials, crushing of eutectic carbides in the material, so that the eutectic carbide non-uniformity ≤ 3; 4 to take the pretreatment of high temperature quenching, and then high temperature tempering Process, through precise temperature control and other measures, can effectively avoid quench cracking caused by high-speed steel raw material metallurgy defects. 2 High-speed steel overheated, over-fired structure High-speed steel overheated, over-sintered structure is characterized by significant grain coarsening, alloying carbides appear sticky, horny, trailing and fully meshed, semi-meshed, or continuous along grain boundaries. Reticulate distribution; partial melting inside the steel structure appears black tissue or eutectic leucite, formed over burned tissue, significantly reduce the intercrystalline bond strength and steel toughness. The main causes of overheating and overheating of high-speed steel are: the quenching heating temperature is too high; the temperature measurement and temperature control instruments are misaligned; when the salt bath furnace is quenched and heated, an error occurs due to radiation on the salt bath surface due to radiation pyrometers; The magnetic switch of the switchboard is out of order; when the tool is heated, it is too close to the electrode or buried in the bottom sediment; there are a large number of angular carbides or carbides in the raw material, and the level of unevenness is too high. High-speed steel overheating, over-burning organizations can easily lead to quenching cracks. Preventive measures are: 1 strictly control the quality of raw materials, eutectic carbide grade should be ≤ 3 ~ 3.5; 2 metallurgical inspection before raw materials into storage and put into operation to ensure that there is no macro-metallurgical defects; 3 before quenching and heating with a test piece High-temperature salt bath furnace, check the relationship between grain size and quenching heating temperature is reasonable (see Table 1); 4 microcomputer temperature control and temperature measurement, temperature measurement accuracy of ± 1.5 °C. Table 1 W6Mo5Cr4V2 high-speed steel carbide grade and overheating quenching heating temperature eutectic carbide non-uniformity grade superheat (grain size 8#) quenching temperature (± 5 °C) ≤ 3 1260 °C 3.5 1250 °C 4.5 1245 °C 7.5 1240 °C 8.5 1230 °C 3 Naphthalene-shaped fracture Naphthalene fracture is a common structural defect of high-speed steel. The fracture is scaly and resembles marble. It has naphthalene luster, rough fracture, and coarse crystal grains (up to Ø1mm). Due to the material's brittleness, low strength and toughness, quenching cracks are easily formed during austenite quenching at high temperature. During hot working such as hot forging, rolling, calendering, etc., austenitizing at 1050～1100°C, thermoplastic deformation at 5%～10% critical deformation, improper forging temperature and repeated annealing without intermediate annealing (or annealing) Insufficient) and other factors are easy to form naphthalene fracture, resulting in quenching cracks. The preventive measures are: 1 Reasonable selection of precision forging temperature, strict control of final forging temperature (≤1000°C), slow cooling after forging, 2 annealing of forging billet prior to quenching, 3 avoidance of 5% to 10% critical deformation, 4 super Grain refinement processing. Taking the above measures can effectively inhibit the formation of high-speed steel naphthalene fractures, to avoid quench cracks. 4 Mechanical design and cold machining cause stress concentration in the tool thickness unevenness, nicks, sharp edges, sharp corners, grooves, holes, bosses and other shape changes caused by notch effect and cold processing surface roughness, deep knife pattern, there are bruises Marking and so on can all lead to stress concentration during quenching of high speed steel tools, thereby inducing quench cracks. If there is a large cold working internal stress before the quenching of the tool (especially the grinding internal stress) is not eliminated, will form a variety of stress superposition when quenching heating and cooling, when the superimposed stress exceeds the material strength limit, will produce quench cracks and distortion . The preventive measures are: 1Improved tool design to make the tool shape reasonable and uniform in thickness. Thick holes can be opened process holes, thin ribs can be increased, deformation can be made of slopes; 2 the tool edges, right angles, sharp corners, chamfered hole; 3 cold-worked surface finish should meet the design requirements to prevent Produce a large knife pattern, use a universal pen to write the mark; 4 eliminate the cold working internal stress through annealing before quenching; 5 use hot bath grading quenching, isothermal quenching and other processes to reduce the organizational stress and thermal stress, to avoid stress concentration. 5 The quenching internal stress and quenching cooling medium high-speed steel's microstructure stress, thermal stress and additional stress are quenching internal stress. When high-temperature steel is austenized and quenched at high temperature, the undercooled austenite is transformed into quenched martensite. Because the former has a small specific volume, the latter has a large specific volume, and the steel reverses from a contracted state to an expanded state, and the internal and external metal phases change. The resulting internal stress caused by changes in specific volume is not tissue stress. The difference between the surface and the center of the large tool and the difference in thickness between the heating and cooling speeds results in a temperature difference, which causes the internal stress generated by volume expansion and contraction to be different from the thermal stress. The internal stress caused by the non-uniformity of the tool surface and the internal structure of the tool and the inconsistent elastic deformation of the tool is an additional stress. When the sum of the above three stresses is greater than the breaking resistance of the material, a quench crack is formed. When the cooling rate of the quenching cooling medium is too large and exceeds the critical quenching cooling rate of the steel, a relatively large quenching internal stress is easily formed, resulting in quenching of the tool. When the cooling rate of the quenching cooling medium is too small and is less than the critical quenching cooling rate of the steel, the desired microstructure properties are not obtained. The minimum cooling rate for quenched martensite transformation is the critical quench cooling rate. High-speed steel has excellent hardenability, and the medium and small cutting tools can be hardened by cooling. However, when isothermal quenching with nitric acid salt occurs, if the nitrate salt contains excess water, the quenching cooling rate may be too high, or when the quenching of the tool is not cooled to room temperature, ie, it is transferred to the water for cleaning, and a large amount of supercooled retained austenite may be highly cold in water. The rapid transformation into quenched martensite, resulting in a large quenching internal stress, resulting in quenching of the tool. The preventive measures are: 1 Quenching medium (such as calcium chloride saturated aqueous solution, C∆-1 organic quenching agent, polyvinyl alcohol) that is quenched at the inflection point of the steel C curve (nose) and slowly cooled below the Ms point of the nose. Aqueous solution, potassium permanganate quenching liquid, etc.) as the ideal quenching cooling medium; 2 using thermal bath (nitrate bath, alkali bath, etc.) graded quenching, isothermal quenching and pretreatment before quenching and other measures to refine the organization, eliminate cold and heat Processing stress can effectively prevent and avoid quench cracking and tool quench distortion. 6 Hydrogen embrittlement HSS tool pickling and electroplating infiltrate the initial hydrogen (H) atoms in the steel into hydrogen molecules (H2) will expand when the high-speed steel cutters are infiltrated, resulting in enormous pressure, resulting in cracks in the grain boundary of the steel, said It is hydrogen embrittlement. Pickling is the chemical reaction of a metal oxide with an acid, which causes the metal oxide to become a soluble salt and leave the metal surface. Quenched HSS has a strong pickling tendency for hydrogen embrittlement. When the tool is usually washed with sulfuric acid or hydrochloric acid, the chemical reaction equation is FeO+H2SO4<====>FeSO4+H2O
FeO+HCl<====>FeCl+H2O
Fe+H2SO4—→FeSO4+H2↑
Fe + HCl - → FeCl + H2 ↑ preventive measures are: 1 acid pickling, such as the generation of excessive primary hydrogen (H), you need to strictly control the acid concentration, temperature and pickling time; 2 after pickling and plating tools Timely washing with water and neutralizing residual acid, and within 4 hours at 190 ~ 200 °C × 2 ~ 4h of low-temperature aging, so that hydrogen can be released, can effectively eliminate hydrogen cracking cracking. 7 The cold-worked high-speed steel cutting tool is austenitized at high temperature, quenched at a critical cooling rate greater than or equal to the steel grade after heat preservation to obtain a quenched martensite structure, but some of the undercooled austenite remains unchanged. Corpus (AR) (about 25% to 35%). If the liquid nitrogen treatment is further performed at -60°C to -160°C, the retained austenite can be transformed into martensite (M). As the specific volume of retained austenite is small, the specific volume of martensite is large, and the expansion of steel occurs, a large secondary quenching phase transformation stress will be generated and superimposed on the primary quenching stress when the superimposed stress is greater than the fracture of the steel. Resistance will result in secondary quenching of the cold treatment. The preventive measures are: 1 Before quenching, quench the knife with boiling water at 100°C for 30 to 40 minutes, or temper at low temperature for 1 hour. Tests show that this method can eliminate 20% to 30% of the quenching internal stress. Because the retained austenite is slightly stable, 2% to 5% can still be retained after cold treatment. Retained austenite is both brittle and tough, absorbs the rapid expansion energy of martensite, relaxes and relieves the stress of phase transformation; 2 After the cold treatment, the tool is put into room temperature water (or hot water) to raise the temperature, eliminating 50%～60 % cold treatment secondary quenching stress; 3 using a number of high temperature tempering and other measures to promote the transformation of retained austenite to martensite, can effectively prevent cold cracking. 8 Grinding cracks High-speed steel grinding cracks often occur during the grinding process. The cracks are fine and shallow (less than 1mm in depth), and are distributed on the surface in a radial network, mostly perpendicular to the grinding direction, similar to quenched mesh cracks. But the reasons for formation are different. When the grinding speed is high, the amount of feed is large, and the cooling is poor, the temperature of the surface metal of the steel part can be increased sharply to the quenching heating temperature, followed by cooling to form secondary quenching of the metal surface layer, resulting in secondary quenching stress; when the material Severe carbide segregation has not been eliminated, or there are many retained austenite in the quenching tool is not transformed, the stress-induced phase transformation is easy to occur during grinding, and the retained austenite is transformed into martensite. The stress of the tissue increases and is superposed with the secondary quenching stress of the grinding process to form a secondary quenching surface grinding crack. The preventive measures are: 1Reducing the grinding speed and feedrate, selecting mild cooling grinding fluid; 2Strictly checking the raw material storage and pre-production inspection, and controlling the eutectic carbide grade of the material (≤3 grade). Those over 3 levels should perform Forging; 3 to avoid high austenitisation quenching heating temperature, the use of computer temperature control, the use of thermal bath quenching, austempering, multiple high temperature tempering and other measures to reduce the organizational stress, thermal stress and the number of retained austenite, etc., Can effectively avoid grinding cracks. 9 WEDM microcracks During spark discharge machining, some of the melted metal remains around the pit of the discharge. Since the EDM is performed in oil or water, the molten metal rapidly cools and solidifies after the pulse discharge ends, and a large tensile stress is generated due to the shrinkage, so that the original stress field is redistributed to form a molten metamorphic layer having a thickness of 0.02 to 0.10 mm. The metamorphic layer is a dendritic as-cast structure, which forms a secondary high-temperature quenching hardened layer after cooling, and generates a large amount of extremely stable retained austenite. The tensile stress caused by the shrinkage of the metamorphic layer is superimposed with the secondary high-temperature quenching stress of the metamorphic layer, and microscopic cracks form on the metamorphic layer, which deepens and enlarges as the electrical parameters of the EDM increase. The preventive measures are as follows: 1 Eliminating the internal stress of the cutter before the WEDM; 2 Strictly controlling the electrical parameters of the wire cutting; 3 Leaving the machining allowance for grinding and polishing, and removing the degenerating layer through subsequent processing; 4 Passing 150~ 200 °C × 2 ~ 4h oil bath to eliminate stress tempering, to prevent the occurrence of micro-cracks EDM. 10 Secondary quenching caused by improper tempering Cracking High-speed steel tools have high-temperature tempering secondary hardening characteristics. After the first martensite quenching, a lot of retained austenite is retained. At high temperature tempering, the retained austenite is transformed into martensite during the tempering and cooling process. If it is rapidly cooled in water or oil, it forms two. Secondary quenching martensite will produce large quenching internal stress; if the flame or high frequency rapid heating is used during tempering, the surface metal will shrink, and the internal part is still martensite, which is inflated due to its large specific volume. So that the surface layer to produce a larger tensile stress, and the first and second quenching stress superposition, resulting in improper secondary tempering due to quenching quenching cracking. Decarburization of the tool surface accelerates the formation of cracks. Preventive measures are: 1 heating tools in a protective atmosphere furnace, vacuum furnace and fully deionized salt bath furnace to prevent oxidative decarburization; 2 quenching tool cooling to the point near the Ms point of the steel into the moderate cooling medium, Should be in the salt bath, alkaline hot bath in graded quenching, isothermal quenching and quenching in the ideal cooling medium; 3 low temperature (≤ 100 °C) into the furnace tempering, slowly warming to ≥ 300 °C after the furnace can be heated to the required Tempering temperature, high temperature tempering After the furnace is cooled to room temperature, the retained austenite (AR)→martensite (M) phase transition is achieved during the tempering and slow cooling process to avoid water cooling and oil cooling and to prevent generation of large Secondary quenching stress. In short, timely tempering after quenching, to prevent quenching stress initiation and expansion; fully tempered, to obtain a stable organization; multiple high-temperature tempering, promote full conversion of retained austenite (AR) → martensite (M), and eliminate two Secondary quenching stress; long time combined tempering, improved fracture toughness and comprehensive mechanical properties can effectively prevent secondary quench cracking caused by improper tempering. 11 Tool Corrosion At present, quenching heating of high-speed steel tool heat treatment process in China is generally performed in a salt bath furnace, and tempering heating is generally performed in a nitrate bath furnace. In addition, pickling must be performed. When the tool is quenched and locally heated, it is in contact with harmful gases such as volatile chlorine gas in the high-temperature salt bath near the upper part of the salt bath surface. It is not only prone to oxidative decarburization, but also leads to band-shaped pitting corrosion with a certain width at the junction of the liquid surface and the air. The preventive measures are: using high-temperature heating and salt-packing method, that is, after the whole tool is salted, the partially quenched heating part is exposed to the liquid surface, so that it is coated with a layer of adhering salt shell, which is isolated from the harmful gases in the air and avoids corrosion. . When a large-scale integral blade is quenched and heated in a high-temperature salt bath furnace, corrosion occurs due to high temperature and long holding time, and it easily reacts with a harmful substance such as iron oxide (FeO) in a salt bath. The preventive measures are: strict implementation of salt bath heating medium heat treatment technical conditions: purity ≥98%, sulfate (BaSO4, Na2SO4, K2SO4) and other impurities ≤0.3%, carbonate (BaCO3, Na2CO3, K2CO3) and other impurities ≤ 0.1 %, water insolubles ≤ 0.1%; each work class must carry out deoxidation and slag removal on the salt bath, simmer once a week, and thoroughly remove impurities in the furnace slag. When the tool is pickled, the nitric acid reacts with the nitrate salt due to excessive or no acid washing, which causes electrochemical corrosion. The preventive measures are: after the pickling of the tool, rinse with flowing clean water twice, and then thoroughly neutralize, and strengthen the sand blasting in time, and stop in the air for no more than 8 hours, and adopt the oil seal method, which can effectively prevent pickling corrosion.
FeO+HCl<====>FeCl+H2O
Fe+H2SO4—→FeSO4+H2↑
Fe + HCl - → FeCl + H2 ↑ preventive measures are: 1 acid pickling, such as the generation of excessive primary hydrogen (H), you need to strictly control the acid concentration, temperature and pickling time; 2 after pickling and plating tools Timely washing with water and neutralizing residual acid, and within 4 hours at 190 ~ 200 °C × 2 ~ 4h of low-temperature aging, so that hydrogen can be released, can effectively eliminate hydrogen cracking cracking. 7 The cold-worked high-speed steel cutting tool is austenitized at high temperature, quenched at a critical cooling rate greater than or equal to the steel grade after heat preservation to obtain a quenched martensite structure, but some of the undercooled austenite remains unchanged. Corpus (AR) (about 25% to 35%). If the liquid nitrogen treatment is further performed at -60°C to -160°C, the retained austenite can be transformed into martensite (M). As the specific volume of retained austenite is small, the specific volume of martensite is large, and the expansion of steel occurs, a large secondary quenching phase transformation stress will be generated and superimposed on the primary quenching stress when the superimposed stress is greater than the fracture of the steel. Resistance will result in secondary quenching of the cold treatment. The preventive measures are: 1 Before quenching, quench the knife with boiling water at 100°C for 30 to 40 minutes, or temper at low temperature for 1 hour. Tests show that this method can eliminate 20% to 30% of the quenching internal stress. Because the retained austenite is slightly stable, 2% to 5% can still be retained after cold treatment. Retained austenite is both brittle and tough, absorbs the rapid expansion energy of martensite, relaxes and relieves the stress of phase transformation; 2 After the cold treatment, the tool is put into room temperature water (or hot water) to raise the temperature, eliminating 50%～60 % cold treatment secondary quenching stress; 3 using a number of high temperature tempering and other measures to promote the transformation of retained austenite to martensite, can effectively prevent cold cracking. 8 Grinding cracks High-speed steel grinding cracks often occur during the grinding process. The cracks are fine and shallow (less than 1mm in depth), and are distributed on the surface in a radial network, mostly perpendicular to the grinding direction, similar to quenched mesh cracks. But the reasons for formation are different. When the grinding speed is high, the amount of feed is large, and the cooling is poor, the temperature of the surface metal of the steel part can be increased sharply to the quenching heating temperature, followed by cooling to form secondary quenching of the metal surface layer, resulting in secondary quenching stress; when the material Severe carbide segregation has not been eliminated, or there are many retained austenite in the quenching tool is not transformed, the stress-induced phase transformation is easy to occur during grinding, and the retained austenite is transformed into martensite. The stress of the tissue increases and is superposed with the secondary quenching stress of the grinding process to form a secondary quenching surface grinding crack. The preventive measures are: 1Reducing the grinding speed and feedrate, selecting mild cooling grinding fluid; 2Strictly checking the raw material storage and pre-production inspection, and controlling the eutectic carbide grade of the material (≤3 grade). Those over 3 levels should perform Forging; 3 to avoid high austenitisation quenching heating temperature, the use of computer temperature control, the use of thermal bath quenching, austempering, multiple high temperature tempering and other measures to reduce the organizational stress, thermal stress and the number of retained austenite, etc., Can effectively avoid grinding cracks. 9 WEDM microcracks During spark discharge machining, some of the melted metal remains around the pit of the discharge. Since the EDM is performed in oil or water, the molten metal rapidly cools and solidifies after the pulse discharge ends, and a large tensile stress is generated due to the shrinkage, so that the original stress field is redistributed to form a molten metamorphic layer having a thickness of 0.02 to 0.10 mm. The metamorphic layer is a dendritic as-cast structure, which forms a secondary high-temperature quenching hardened layer after cooling, and generates a large amount of extremely stable retained austenite. The tensile stress caused by the shrinkage of the metamorphic layer is superimposed with the secondary high-temperature quenching stress of the metamorphic layer, and microscopic cracks form on the metamorphic layer, which deepens and enlarges as the electrical parameters of the EDM increase. The preventive measures are as follows: 1 Eliminating the internal stress of the cutter before the WEDM; 2 Strictly controlling the electrical parameters of the wire cutting; 3 Leaving the machining allowance for grinding and polishing, and removing the degenerating layer through subsequent processing; 4 Passing 150~ 200 °C × 2 ~ 4h oil bath to eliminate stress tempering, to prevent the occurrence of micro-cracks EDM. 10 Secondary quenching caused by improper tempering Cracking High-speed steel tools have high-temperature tempering secondary hardening characteristics. After the first martensite quenching, a lot of retained austenite is retained. At high temperature tempering, the retained austenite is transformed into martensite during the tempering and cooling process. If it is rapidly cooled in water or oil, it forms two. Secondary quenching martensite will produce large quenching internal stress; if the flame or high frequency rapid heating is used during tempering, the surface metal will shrink, and the internal part is still martensite, which is inflated due to its large specific volume. So that the surface layer to produce a larger tensile stress, and the first and second quenching stress superposition, resulting in improper secondary tempering due to quenching quenching cracking. Decarburization of the tool surface accelerates the formation of cracks. Preventive measures are: 1 heating tools in a protective atmosphere furnace, vacuum furnace and fully deionized salt bath furnace to prevent oxidative decarburization; 2 quenching tool cooling to the point near the Ms point of the steel into the moderate cooling medium, Should be in the salt bath, alkaline hot bath in graded quenching, isothermal quenching and quenching in the ideal cooling medium; 3 low temperature (≤ 100 °C) into the furnace tempering, slowly warming to ≥ 300 °C after the furnace can be heated to the required Tempering temperature, high temperature tempering After the furnace is cooled to room temperature, the retained austenite (AR)→martensite (M) phase transition is achieved during the tempering and slow cooling process to avoid water cooling and oil cooling and to prevent generation of large Secondary quenching stress. In short, timely tempering after quenching, to prevent quenching stress initiation and expansion; fully tempered, to obtain a stable organization; multiple high-temperature tempering, promote full conversion of retained austenite (AR) → martensite (M), and eliminate two Secondary quenching stress; long time combined tempering, improved fracture toughness and comprehensive mechanical properties can effectively prevent secondary quench cracking caused by improper tempering. 11 Tool Corrosion At present, quenching heating of high-speed steel tool heat treatment process in China is generally performed in a salt bath furnace, and tempering heating is generally performed in a nitrate bath furnace. In addition, pickling must be performed. When the tool is quenched and locally heated, it is in contact with harmful gases such as volatile chlorine gas in the high-temperature salt bath near the upper part of the salt bath surface. It is not only prone to oxidative decarburization, but also leads to band-shaped pitting corrosion with a certain width at the junction of the liquid surface and the air. The preventive measures are: using high-temperature heating and salt-packing method, that is, after the whole tool is salted, the partially quenched heating part is exposed to the liquid surface, so that it is coated with a layer of adhering salt shell, which is isolated from the harmful gases in the air and avoids corrosion. . When a large-scale integral blade is quenched and heated in a high-temperature salt bath furnace, corrosion occurs due to high temperature and long holding time, and it easily reacts with a harmful substance such as iron oxide (FeO) in a salt bath. The preventive measures are: strict implementation of salt bath heating medium heat treatment technical conditions: purity ≥98%, sulfate (BaSO4, Na2SO4, K2SO4) and other impurities ≤0.3%, carbonate (BaCO3, Na2CO3, K2CO3) and other impurities ≤ 0.1 %, water insolubles ≤ 0.1%; each work class must carry out deoxidation and slag removal on the salt bath, simmer once a week, and thoroughly remove impurities in the furnace slag. When the tool is pickled, the nitric acid reacts with the nitrate salt due to excessive or no acid washing, which causes electrochemical corrosion. The preventive measures are: after the pickling of the tool, rinse with flowing clean water twice, and then thoroughly neutralize, and strengthen the sand blasting in time, and stop in the air for no more than 8 hours, and adopt the oil seal method, which can effectively prevent pickling corrosion.