Common defects in heat treatment and preventive measures
Time：2022-06-06 08:39:17 / Popularity： / Source：
1. Unqualified hardness
Hardness of metal materials has a certain empirical relationship with its static tensile strength and fatigue strength, and has a certain degree of relationship with processing properties such as cold formability, machinability and welding performance of metal; hardness test does not damage workpiece, test is simple, and data is intuitive, so it is widely used as the most important quality inspection index for heat-treated workpieces, and many workpieces are still the only technical requirements.Unacceptable hardness is one of the most common heat treatment defects. Mainly manifested as insufficient hardness, insufficient quenching cooling rate, surface decarburization, insufficient hardenability of steel, excessive retained austenite after quenching, insufficient tempering and other factors. Phenomenon of low hardness in local area of quenched workpiece is called soft spot.
Microstructure in soft spot area is mostly a mixed structure of martensite and trotenite distributed along prior austenite grain boundaries. Soft spots or uneven hardness are usually caused by uneven quench heating or uneven quench cooling. Uneven furnace temperature during heating and insufficient heating temperature or holding time are main reasons for uneven heating.
Uneven cooling is mainly caused by bubbles of quenching medium adhering to surface of workpiece during quenching and cooling, quenching medium being polluted (for example, there are oil suspended beads in the water) or insufficient stirring of quenching medium. In addition, steel structure is too coarse, there is serious segregation, and large carbides or large free ferrite will also cause uneven quenching to form soft spots.
1.1 Soft spot
Purpose of quenching heating is to complete structural transformation of workpiece during quenching process. For this purpose, it must be heated to an appropriate temperature and have sufficient holding time. Low heating temperature and insufficient holding time make original pearlite structure not completely transformed into austenite and transformed austenite composition is not uniform, complete martensite structure cannot be obtained after quenching, resulting in formation of soft spots after quenching of workpiece. .Figure 1 shows microstructure of a hand tap made of T12 steel due to insufficient heating: fine needle martensite + quenched trotenite + pearlite. Performance is manifested as uneven hardness.
1-fine needle martensite 2-quenched trotenite 3-pearlite
Insufficient stirring of quenching medium, insufficient movement of workpiece in quenching medium, or wrong direction of workpiece entering medium often delays rupture of vapor film on some parts of workpiece surface, resulting in a decrease in cooling rate there, resulting in emergence of high-temperature decomposition products, formation of soft spots or a local decrease in hardness. Water vapor films are more stable than salt water, so soft spots are more likely to form on water-quenched workpieces. The higher temperature of water and aqueous solutions, the more prone to soft spots.
Carbon steel with poor hardenability is prone to soft spots when workpiece section is large. If surface of workpiece is not clean, such as rust, carbon black, etc., it will also cause phenomenon of low hardness after quenching.
1.2 Insufficient hardness
Insufficient heating often results in insufficient hardness of quenched part. Improper cooling, however, is a common cause of insufficient workpiece hardness. After workpiece is released from furnace and before quenching, pre-cooling time is too long, cooling medium is improperly selected or temperature of cooling medium is controlled too high, resulting in insufficient cooling capacity, oxide scale or salt attached to surface of workpiece, temperature of workpiece is too high when workpiece is removed from quenching medium after quenching, which may cause decomposition of supercooled austenite in pearlite transformation region of C curve, formation of non-martensitic structures such as sorbite and trotenite, resulting in insufficient hardness of workpiece.Existence of a large amount of retained austenite in quenched structure is an important reason for insufficient hardness of quenched workpiece. Amount of retained austenite is related to chemical composition of austenite. When carbon content is greater than 0.5%~0.6%, existence of retained austenite can be clearly observed in quenched structure. Continue to increase carbon content and amount of retained austenite A sharp rise, when mass fraction of carbon is 1.4%, amount of retained austenite (volume fraction) reaches 30%.
Any alloying elements that are solid-dissolved in austenite by substitution will cause an increase in the amount of retained austenite. When amount of retained austenite is small, there is no obvious effect on hardness. When amount of retained austenite is large, hardness will decrease. Retained austenite with a volume fraction of 20% will decrease quenching hardness by about 6.5HRC.
1.3 Insufficient soft spots and hardness of high-frequency quenching and carburizing workpieces
Soft spots of high-frequency quenched workpieces include two kinds of residual soft spots that are not hardened locally on the surface and deep soft spots with uneven depth of hardened layer. These hardness defects are caused by factors such as improper material selection, poor original structure, electrical parameters of high-frequency quenching heating, improper inductors and cooling devices.High-frequency quenching is mostly used for medium-carbon structural steel and low-carbon medium-alloy structural steel. Since high-frequency quenching heating is rapid heating, carbon in austenite is too late to diffuse and is not fully homogenized. Therefore, steels containing carbide-forming elements such as Cr, Mo, W, V, etc., are prone to soft spots and uneven hardness during high-frequency induction heating quenching due to their high transformation point. When selecting steels for high-frequency quenching, above elements should not exceed a certain content.
Type, shape, size and distribution of carbides in steel have a significant impact on quality of induction hardened workpieces. When there are network carbides in steel, carbides are too large in size and unevenly distributed, defects such as uneven hardness and insufficient hardness are easy to occur. Therefore, high frequency quenching is greatly affected by pre-heat treatment, the best original structure of high frequency quenching is tempered sorbite treated by quenching and tempering.
When high-frequency induction coil is uneven, it will also lead to insufficient quenching hardness, improper injection angle, unreasonable size and number of injection holes, or blocked injection holes, which often lead to insufficient hardness of high-frequency quenched workpieces or formation of soft spots.
Insufficient hardness and soft spots of carburized workpieces are caused by insufficient carburization, decarburization during quenching, too low quenching temperature, insufficient quenching cooling rate, excessive residual austenite on the surface, excessive tempering, unclean workpiece surface, uneven carburization or uneven cooling.
2. Mechanical properties of non-ferrous metal alloys are unqualified
The most widely used non-ferrous metals in industry are aluminum, copper, magnesium, titanium and their alloys. Heat treatment principle of non-ferrous metals and steel is same, but has its own characteristics. For example, eutectoid transformation plays an important role in heat treatment of steel, but it is rarely encountered in non-ferrous metals; martensitic transformation is main means by which steel materials are strengthened, but except for a few copper alloys and titanium alloys, other non-ferrous metals generally cannot be strengthened by martensitic transformation.Commonly used heat treatment processes for non-ferrous metals are homogenization annealing, recrystallization annealing, stress relief annealing, solution treatment and aging treatment. Solution aging is the most commonly used and most important heat treatment strengthening process for non-ferrous metals.
1) Non-ferrous metals are lively and have strict requirements on heating environment. For example, heating environment of titanium alloy should generally be a vacuum or a slightly oxidizing atmosphere; to avoid oxidation, magnesium alloys are often heated in a protective atmosphere of sulfur dioxide or carbon dioxide; to avoid hydrogen embrittlement, red copper needs to be heat-treated in a neutral or weakly oxidizing atmosphere.
2) In order to achieve maximum solid solution effect, solid solution temperature of many non-ferrous metal alloys is close to temperature of solidus. In order to prevent overheating and overburning, furnace temperature and heating holding time must be strictly controlled.
Common causes of unqualified mechanical properties of non-ferrous metals due to improper heat treatment and prevention methods are shown in Table 3.
▼Table 3 Common mechanical property defects and prevention methods of non-ferrous metal heat treatment
| Copper alloy | Copper Oxygen Brittle | Intergranular brittle fracture during tensile test | Oxygen-containing copper is heat-treated in hydrogen or a reducing atmosphere containing hydrogen. Hydrogen reduces cuprous oxide and generates high-pressure water vapor to cause intergranular fracture. | Heat treatment in medium or weakly oxidizing atmosphere |
| Unqualified mechanical properties | Uneven hardness | Uneven furnace temperature, instrument failure; too much charging | Control furnace temperature, replace instrument; control furnace load | |
| Insufficient quenching, high hardness, low plasticity | Solution temperature is too low, holding time is insufficient, quenching transfer time is too long, and temperature of quenching medium is too high | Adjust heat treatment process parameters and use flowing water | ||
| Overheating and overburning, high brittleness | Gauge malfunctioning; furnace temperature too high; when protected with charcoal, charcoal burning increases temperature | Check and replace instrument; adjust process parameters; when protected with charcoal, put it in a sealed box, add 10% NA2SO4 to the charcoal | ||
| Magnesium alloy | Unqualified mechanical properties | Uneven performance | Furnace temperature is not uniform; cooling speed of workpiece is not uniform; thickness of each part of workpiece is not uniform, and holding time is insufficient; grain deformity grows | Improve equipment, control furnace temperature uniformity; extend holding time; perform stress relief treatment before heat treatment, select appropriate cold iron during casting, and use intermittent heating method during solution treatment |
3. Overheating and overburning
When a metal or alloy is heated during heat treatment, due to excessively high temperature, grains grow very large, so that performance is significantly reduced, which is called overheating; when heating temperature is close to its solidus, grain boundary oxidizes and begins to partially melt, which is called overburning.3.1 Overheating
Superheated structure includes coarse grains of structural steel, coarse martensite, excessive retained austenite, Widmanstatten structure, reticulated carbide, eutectic structure (ledeburite structure), naphthalene-like fracture of high-speed steel, too much ferrite in martensitic stainless steel, dezincification of brass alloy causes white ash on the surface, and pockmarked surface after pickling.A typical superheated structure is shown in Figure 7. Overheating structure can be divided into two categories: stable overheating and unstable overheating according to difficulty of eliminating it by normal heat treatment process. Generally, overheating structure can be eliminated by normal heat treatment, which is called unstable overheating structure. Stable overheated structure refers to overheated structure that cannot be completely eliminated by normal normalizing, annealing and quenching.
Important feature of overheating is coarse grains, which will reduce yield strength, plasticity, impact toughness and fatigue strength of steel, and increase brittle transition temperature of steel. See Figure 8, Figure 9, Table 5, Table 6.
| Average grain diameter*100/mm | Db/MPa | δ(%) |
| 9.7 | 163 | 28.8 |
| 7.0 | 184 | 30.6 |
| 2.5 | 215 | 39.5 |
| Grain size class | Fatigue limit at room temperature/MPa | 700℃ fatigue limit/MPa |
| 4-6 | 290 | 400 |
| 7-9 | 400 | 590 |
Another important feature of overheating is coarse quenched martensite, which will reduce impact toughness and wear resistance, increase tendency of quenching deformation and quenching cracking, as shown in Figure 10, Figure 11, and Tables 7 and 8. Medium carbon steel is immediately divided into 8 grades according to its shape and size, grades 7 and 8 are superheated structures.
| Test type | Material | Martensitic grade | |||||||
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | ||
| Dimensional change of ring specimen 8+8.02 | 45 # steel | +4.5 | +5.3 | +7.3 | +9.2 | +10.1 | +12 | +12 | |
| 40Cr | +6.4 | +6.2 | +5.1 | +4 | +5.9 | +6.4 | +8.4 | ||
| Maximum radial runout of circular specimens | 45 # steel | 7.8 | 12.8 | 13.7 | 22.7 | 31.7 | 38.3 | 46.3 | |
| 40Cr | 1.8 | 5.5 | 9.3 | 10.7 | 2.2 | 7.6 | 13.7 | ||
| Martensitic grade | Quenching times of long specimens | Quenching times of round specimens | ||||||||
| 1 | 2 | 3 | 4 | 5 | 1 | 2 | 3 | 4 | 5 | |
| 3 | 3 | |||||||||
| 4 | 1 | 2 | ||||||||
| 5 | 1 | |||||||||
| 6 | ||||||||||
| 7 | 2 | 3 | 3 | |||||||
| 8 | 4 | 1 | 3 | 7 | ||||||
Overheating defects of steel include Widmanners microstructure, naphthalene-shaped fracture, stone-shaped fracture, etc., which can not only greatly reduce mechanical properties of steel, but also easily cause quenching cracking.
Characteristics and preventive measures of various overheated tissues are listed in Table 9. In order to prevent overheating, a reasonable heat treatment process should be correctly formulated and implemented, furnace temperature and holding time should be strictly controlled. Generally, overheated structure can be eliminated by multiple annealing or normalizing. For more serious overheated structures, such as stone fractures, heat treatment cannot be used. To be eliminated, it must be eliminated by combined action of high temperature deformation and annealing.
▼Table 8 Organizational characteristics and preventive measures
| Name | Main features | Preventive measures |
| Coarse grains | Austenite grain size below grade 3 | 1. To prevent overheating, strictly control furnace temperature and holding time, reduce heating speed or stage heating 2. Eliminated by multiple normalizing or annealing 3. Stone-like fracture cannot be eliminated by ordinary heat treatment. Grain must be refined by high temperature deformation, and then eliminated by annealing. |
| Coarse martensite | Martensitic lath or needle longer in grades 7-8 | |
| Too much retained austenite | Residual austenite in the quenched structure of multi-grade steel with carbon content and alloying elements | |
| Wei's organization | Ferrite of hypoeutectoid steel is precipitated at austenite grain boundary and cleavage plane, showing a fine mesh structure | |
| Reticulated carbide | Super-Eutectoid Steel Appears Reticular Carbide Distributed Along Grain Boundaries in Microstructure | |
| Graphitization (black brittle) | In annealed structure of high carbon steel, some cementite is transformed into graphite, and fracture is gray-black | |
| Eutectic structure | Eutectic ledeburite appears in high-speed steel overheating | |
| Naphthalene fracture | There are many different orientations on fracture, relatively smooth facets, shining like naphthalene-like crystals | |
| Stone fracture | On fiber fracture matrix, there are different orientations, no metallic luster, and gray-white granular sections. | |
| Too much delta ferrite | CR3 stainless steel is overheated and has a large amount of delta ferrite in structure |
3.2 Overheating
Overfired structure includes local melting of grain boundaries, fiber voids, blackening and blistering on surface of aluminum alloys, gray and dull fracture surfaces, oxide tumors on surface of magnesium alloys.A typical overburned structure is shown in Figure 12.
Over-burned structure seriously deteriorates performance and is prone to heat treatment cracks. Therefore, over-burning is a heat-treatment defect that is not allowed. Once over-burning occurs, precious parts can only be scrapped. Therefore, over-burning should be strictly prevented during heat-treatment production.
4. Unqualified spheroidization level
Automobiles, tractors and various other machines use a lot of standard parts and fasteners. Most standard parts such as shafts, pins, and rods are turned by automatic lathes, while most fasteners such as bolts, nuts, and rivets are processed by cold heading. In order to improve productivity and adapt to automatic cutting and cold heading processing, preparatory heat treatment of steel is annealing or spheroidizing annealing, degree of spheroidization should be controlled.Automatic turning requires steel to have good turning performance, and plasticity should not be too high, otherwise it will be easy to "stick to knife" and turning chips will continue. It is hoped that steel structure is flaky pearlite; while cold heading processing requires steel to have good cold heading performance and plasticity to ensure no cracking during cold heading, and it is hoped that metallographic structure of steel is spherical pearlite.
For this reason, there is an industry standard JB/T5074-91 "Spheroidization Rate Level of Low and Medium Carbon Steel" to evaluate spheroidization level. According to grade of carbide spheroidization, spheroidization rate of grade 1 is 0, that is, pearlite is completely flaky, and spheroidization rate of grade 6 is 100%, that is, carbide is in a complete global state. Medium carbon steel for cold heading generally requires grade 4 to 6, low and medium carbon steel for automatic machine tool processing generally requires grade 1 to 3.
Unqualified spheroidization of low and medium carbon steel preparatory heat treatment will seriously affect its cold heading and automatic cutting performance. Influence of spheroidization grade on cold heading performance is shown in Table 10. It can be seen that when amount of spheroidization grades 1~3 is large, cracking will occur, while in grades 4~6, there will be no cracking during cold heading, so spheroidization rate of cold heading steel is controlled at grade 4~6 to be qualified. Large-scale production practice shows that when spheroidization rate of steel used for automatic turning is controlled at 1~3 grades, surface roughness of parts is suitable, band saw wear is normal, and productivity is high; if spheroidization rate exceeds 3 grades, it is difficult to carry out automatic machine tool processing.
Table 10 Influence of different grades of metallographic structure on results of pre-upset forging
| Spheroidization level Deformation |
1 | 2 | 3 | 4 | 5 | 6 |
| Cold top forging reduction 1/3H (Deformation amount is 66.7%) |
1 out of 3 samples cracked | Good | Good | Good | Good | Good |
| Cold top forging reduction 1/4H (Deformation amount is 72.2%) |
2 out of 3 samples cracked | 2 out of 3 samples cracked | 1 out of 3 samples cracked | Good | Good | Good |
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