Cause Analysis and Preventive Measures of Quenching Crack of High Speed ​​Steel Tool
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1 Metallurgical defects of high speed steel raw materials
The large amount of carbide contained in high speed steel is hard and brittle and is a brittle phase. The primary eutectic carbide is distributed in the thick matrix (or dendritic) in the steel matrix. After the billet is rolled and rolled by the billet, although the alloy carbide has a certain degree of fracture and refinement, the segregation of carbide still exists, and it is distributed in the strip, full mesh, semi-mesh or piled along the rolling direction. . Carbness non-uniformity increases as the diameter or thickness of the raw material increases. The eutectic carbide is quite stable, and conventional heat treatment is difficult to eliminate, which can cause stress concentration and become a source of quenching cracks. Segregation or exceeding the standard of sulfur, phosphorus and other impurities in steel is also an important cause of quenching. High-speed steel has poor thermal conductivity and thermoplasticity, and has large deformation resistance. When hot working, it tends to cause micro-cracks in the surface layer and inner layer of the metal, and finally the material is scrapped due to crack propagation during quenching. Macroscopic metallurgical defects formed by large steel ingots during hot processing such as smelting, rolling or forging, such as looseness, shrinkage, bubbles, segregation, white spots, dendrites, coarse crystals, inclusions, internal cracks, hairline, and large particle carbonization Both physical and non-metallic slag inclusions tend to cause stress concentration during quenching, and quenching cracks occur when the stress is greater than the material strength limit.
The precautionary measures are as follows: (1) using small steel ingots to open and roll various kinds of tool raw materials; 2 using secondary refining electroslag remelting steel ingots, which have high purity, less impurities, fine grains, small carbides, uniform structure, no Advantages of macroscopic metallurgical defects; 3 forging and forging unqualified raw materials, crushing eutectic carbides in the material, making the eutectic carbide non-uniformity ≤ 3; 4 adopting high temperature step quenching, high temperature tempering pretreatment The process, through precise temperature control and other measures, can effectively avoid quenching cracks caused by metallurgical defects of high-speed steel raw materials.
2 high temperature steel overheated, over burned tissue
The superheated and over-fired structure of high-speed steel is characterized by coarse grain coarsening, alloy carbides appearing adhesion, horns, trailing and full-net, semi-mesh or continuous network distribution along the grain boundary; Melting occurs in black tissue or eutectic Leysite, forming a burnt structure, which significantly reduces intergranular bonding and steel toughness. The main causes of overheating and overburning of high-speed steel are: quenching heating temperature is too high, temperature measurement and temperature control instrument misalignment; salt bath furnace quenching heating, due to salt bath surface smoke caused by radiation pyrometer temperature error; The pressure switchboard magnetic switch fails; the tool is too close to the electrode when heated or buried in the bottom sediment; the raw material has a large amount of horny carbide or carbide unevenness level is too high. High-temperature steel overheating and over-burning structure can easily lead to quenching cracks.
The precautionary measures are as follows: 1 strictly control the quality of raw materials, eutectic carbide grade should be ≤ 3 ~ 3.5; 2 metallurgical inspection before raw materials storage and production, to ensure no macro metallurgical defects; Check the high temperature salt bath furnace, check whether the relationship between grain grade and quenching heating temperature is reasonable (see the table below); 4 using microcomputer temperature control and temperature measurement, the temperature measurement accuracy reaches ± 1.5 °C. Eutectic carbide
Unevenness level
Overheating (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 fracture
The naphthalene fracture is a common structural defect of high speed steel. The fracture is fish scale, resembling marble, with the luster of naphthalene, the fracture is extremely rough, and the grain is coarse (up to 1 mm). Due to the large brittleness of the material, the toughness is low, and quenching cracks are easily formed during high-temperature austenitizing quenching. In hot forging, rolling, calendering, etc., austenitizing at a temperature of 1050 to 1100 °C, thermoplastic deformation at 5% to 10% critical deformation, improper forging temperature and repeated quenching without intermediate annealing (or annealing) Factors such as insufficient) are easy to form a naphthalene fracture, resulting in quenching cracks.
The precautionary measures are as follows: (1) Reasonably select the precision forging temperature, strictly control the final forging temperature (≤1000 °C), slow cooling after forging; 2 should be fully annealed before quenching; 3 avoid critical deformation at 5% to 10%; Grain refining treatment, etc. Taking the above measures can effectively inhibit the formation of high-speed steel naphthalene fractures and avoid quenching cracks.
4 Mechanical design and improper cold working cause stress concentration
Uneven thickness of the tool, cracks due to sudden changes in shape such as corners, sharp edges, sharp corners, grooves, holes, bosses, and rough surface roughness, deep kerf, bruises, and marking can cause high-speed steel The stress concentration during the quenching of the tool induces quenching cracks. If there is a large cold working internal stress (especially grinding internal stress) before the quenching of the tool is not eliminated, a variety of stress superposition will be formed during quenching heating and cooling. When the superimposed stress exceeds the material strength limit, quenching crack and distortion will occur. .
The precautionary measures are as follows: (1) Improve the tool design so that the shape of the tool is reasonable and the thickness is uniform. The thick hole can open the process hole, the thin part can increase the rib, the deformation can be made into the slope; 2 the edge, right angle and sharp corner of the tool are rounded, and the hole is chamfered; 3 cold-finished surface finish should meet the design requirements to prevent Produce coarse knives, write marks with universal pen; 4 eliminate cold working internal stress by annealing before quenching; 5 use thermal bath grading quenching, austempering and other processes to reduce tissue stress and thermal stress, to avoid stress concentration.
5 quenching internal stress and quenching cooling medium
The microstructure stress, thermal stress and additional stress of high speed steel are quenching internal stresses. When the high-speed steel is subjected to high-temperature austenitizing quenching, the supercooled austenite transforms into quenched martensite. Because the former has a small specific volume, the latter has a large specific volume, and the steel is reversed from a contracted state to a expanded state, and the inner and outer layers of the metal are transformed. The internal stress caused by the change in the specific volume change is the tissue stress. The surface and the center of the large tool and the difference in thickness are inconsistent with the heating and cooling rates to form a temperature difference, which causes the internal stress generated by the difference in volume expansion and contraction to be thermal stress. The internal stress caused by the uneven surface of the tool and the internal structure and the inconsistent elastic deformation of the tool are additional stresses. 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, it is easy to form a large quenching internal stress, which causes the tool to be quenched. When the cooling rate of the quenching cooling medium is too small, less than the critical quenching cooling rate of the steel, the desired microstructure properties are not obtained. The minimum cooling rate at which the quenched martensite transformation is obtained is the critical quenching cooling rate. High-speed steel has excellent hardenability, and small and medium-sized tools can be hardened by air cooling. However, when austempering with nitrate salt, such as excessive water in the nitrate salt, the quenching cooling rate may be too large, or when the tool is quenched and not cooled to room temperature, it is transferred to water for cleaning, which can make a large amount of supercooled retained austenite cold in water. The speed changes to quenched martensite, which produces large quenching internal stresses, causing the tool to be quenched.
The precautionary measures are as follows: (1) Select a quenching medium (such as a saturated aqueous solution of calcium chloride, C?-1 organic quenching agent, polyvinyl alcohol) which is rapidly cooled at the inflection point of the C curve of the steel (nose) and slowly cooled below the Ms point of the nose. Aqueous solution, potassium permanganate quenching liquid, etc.) as an ideal quenching cooling medium; 2 using thermal bath (nitrate bath, alkali bath, etc.), step quenching, austempering and pre-quenching pretreatment, refining the structure, eliminating cold and heat Machining stress can effectively prevent and avoid quenching and tool quenching distortion.
6 hydrogen embrittlement
When the high-speed steel tool is pickled and the initial ecological hydrogen (H) atoms invaded into the steel are converted into hydrogen molecules (H2) during electroplating, expansion will occur, causing tremendous pressure, causing cracks in the grain boundary of the steel, called hydrogen embrittlement. . Pickling is the chemical reaction of a metal oxide with an acid that causes the metal oxide to become a soluble salt and detach from the metal surface. Quenched high speed steel has a strong tendency to pickle hydrogen cracking. 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↑
The precautionary measures are as follows: 1 When pickling, if excessive nascent hydrogen atoms (H) are produced, the acid concentration, temperature and pickling time should be strictly controlled; 2 the tool should be rinsed and neutralized with clean water after pickling and plating. The acid is tempered at 190-200 ° C for 2 to 4 hours in 4 hours to release hydrogen gas, which can effectively eliminate hydrogen embrittlement cracking.
7 cold treatment crack
The high-speed steel tool is austenitized at high temperature, and is quenched at a critical cooling rate greater than or equal to the steel to obtain a quenched martensite structure, but some of the supercooled austenite is not transformed and becomes retained austenite ( AR) (about 25% to 35%). If the liquid nitrogen cold treatment is further carried out at -60 ° C to -160 ° C, the retained austenite can be transformed into martensite (M). Since the retained austenite specific volume is small, the specific volume of martensite is large, and the steel member expands, a large secondary quenching phase transformation microstructure stress will be generated and superimposed with the primary quenching stress, when the superimposed stress is greater than the breaking of the steel species. Resistance, it will produce cold treatment secondary quenching.
The precautions are as follows: 1 Before quenching, the quenching tool is boiled in 100 ° C boiling water for 30 to 40 minutes, or tempered at low temperature for 1 hour. Tests have shown that this method can eliminate the quenching internal stress of 20% to 30%. Since the retained austenite is slightly stable, it can remain 2% to 5% after cold treatment. The retained austenite is brittle and tough, absorbs the rapid expansion energy of martensite, relaxes and relaxes the phase transformation stress; 2 after the cold treatment, the tool is placed in room temperature water (or hot water) to raise the temperature, which can eliminate 50%-60 % cold treatment secondary quenching stress; 3 using multiple high temperature tempering and other measures to promote the transformation of retained austenite to martensite, can effectively prevent cold cracking.
8 grinding crack
High-speed steel grinding cracks often occur during the grinding process. The cracks are fine and shallow (depth is less than 1mm), and they are distributed on the surface in a radial network. Most of them are perpendicular to the grinding direction, similar to quenching network cracks, but the causes are different. . When the grinding speed is high, the feed rate is large, and the cooling is poor, the surface metal temperature of the steel piece can be sharply increased to the quenching heating temperature, and then the cooling is formed to form a secondary surface quenching of the metal surface to generate secondary quenching stress; Serious carbide segregation is not eliminated, or more retained austenite is not transformed in the quenching tool, and stress-induced phase transformation is prone to occur during grinding, which causes the retained austenite to transform into martensite. The microstructure stress increases and is superimposed with the secondary quenching stress of the grinding process to form a secondary quenching surface grinding crack.
The precautionary measures are as follows: (1) reduce the grinding speed and feed rate, and select the tempering grinding coolant; 2 strictly check the raw materials and check before the production, and control the eutectic carbide grade of the material (≤3). Change forging; 3 to avoid excessive austenitizing quenching heating temperature, using computer temperature control, using thermal bath grading quenching, austempering, multiple high temperature tempering and other measures to reduce tissue stress, thermal stress and the amount of retained austenite, etc. It can effectively avoid grinding cracks.
9 Electric discharge wire cutting micro crack
During spark discharge machining, a portion of the molten metal remains around the etch pit at the discharge point. Since the electric discharge machining is performed in oil or water, the molten metal rapidly cools and solidifies after the end of the pulse discharge, 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 quench hardened layer after cooling, and generates a large amount of extremely stable retained austenite. The tensile stress generated by the shrinkage of the metamorphic layer is superimposed on the secondary high-temperature quenching stress of the metamorphic layer, and microcracks are formed on the metamorphic layer, and deepen and expand with the increase of electrical parameters of the electrical discharge machining.
The precautionary measures are as follows: (1) The internal stress of the tool should be completely eliminated before the WEDM machining; 2 The electrical parameters of the wire cutting should be strictly controlled; 3 The machining allowance for grinding and polishing should be left, and the metamorphic layer should be removed by subsequent processing; 200 ° C × 2 ~ 4h oil bath to eliminate stress tempering, to prevent micro-cracks during EDM.
10 improper tempering caused secondary quenching crack
High speed steel tools have high temperature tempering secondary hardening characteristics. After the first martensite quenching, more retained austenite is retained. During high temperature tempering, the retained austenite phase changes to martensite during tempering and cooling. If it is rapidly cooled in water or oil, it forms two. When quenching martensite, it will produce larger quenching internal stress; if it is tempered by flame or high frequency rapid heating, the surface metal will shrink, while the interior is still martensite structure, which is inflated due to large specific volume. Therefore, the surface layer generates a large tensile stress, which is superimposed with the primary and secondary quenching stresses, resulting in secondary hardening and quenching cracks caused by improper tempering. Decarburization of the tool surface accelerates the formation of cracks.
The precautions are as follows: (1) heating the tool in a protective atmosphere furnace, a vacuum electric furnace and a salt bath furnace with sufficient deoxidation to prevent oxidative decarburization; 2 when the quenching tool is cooled to the vicinity of the Ms point of the steel, it is taken out into the tempering cooling medium, It should be classified and quenched in the salt bath hot bath, alkali heat bath, austempering and quenching in the ideal cooling medium; 3 low temperature (≤100 °C) into the furnace tempering, slowly heating up to ≥300 °C, can be heated with the furnace to the required Tempering temperature, high temperature tempering and heat preservation, then air cooling to room temperature, in the process of tempering slow cooling to achieve residual austenite (AR) → martensite (M) phase change, to avoid water cooling, oil cooling, to prevent the emergence of larger two Secondary quenching stress. In short, tempering in time after quenching, to prevent the initiation and expansion of quenching stress; full tempering, to obtain stable structure; multiple high temperature tempering, promote the full transformation of retained austenite (AR) → martensite (M), and eliminate two Sub-quenching stress; long-term and combined tempering, improved fracture toughness and comprehensive mechanical properties can effectively prevent secondary quenching cracks caused by improper tempering.
11 tool corrosion
At present, the quenching heating of the high-speed steel tool heat treatment process in China is generally carried out in a salt bath furnace, and the tempering heating is generally carried out in a salt bath furnace. In addition, pickling must be carried out.
When the tool is quenched locally, the contact with the harmful gas such as chlorine gas in the high-temperature salt bath near the salt bath surface is not only prone to oxidative decarburization, but also causes a certain width of strip-like pitting corrosion at the junction between the liquid surface and the air. The precautionary measures are: using high temperature heating salt method, that is, after the whole tool is put into the salt, the local non-quenching heating part is exposed to the liquid surface, so that it is covered with a layer of adhered salt shell, which is isolated from the harmful gas in the air to avoid corrosion. .
When the large-scale whole blade is quenched and heated in a high-temperature salt bath furnace, it is easy to react with harmful substances such as iron oxide (FeO) in the salt bath due to high temperature and long holding time. The precautionary measures are: strict implementation of salt bath heating medium heat treatment technical conditions: purity ≥ 98%, sulfate (BaSO4, Na2SO4, K2SO4) and other impurities content ≤ 0.3%, carbonate (BaCO3, Na2CO3, K2CO3) and other impurities ≤ 0.1 %, water insoluble matter ≤ 0.1%; each work class must be deoxidized and slag on the salt bath, once a week, thoroughly remove the slag impurities in the furnace.
When the tool is pickled, the acid is excessively reacted or the residual acid is not washed, so that the chemical reaction between nitric acid and the nitrate salt causes electrochemical corrosion. The precautionary measures are: after pickling the tool, rinse it twice with running water, thoroughly neutralize it, and blast it in time, and park it in the air for no more than 8 hours, and take the oil seal to prevent acid corrosion.