Delayed fracture is a common defect in the use of wave washers. Recent studies have shown that delayed fracture mainly occurs in tempered martensitic steel. The conditions for delayed fracture are:
①Use stress ≥1200Mpa;
②The fracture occurs at the place where the tensile stress is the greatest, and the compressive stress generally does not produce delayed fracture;
③The tempering at around 350℃ often leads to increased delayed fracture sensitivity of wave washers; ④Delayed fracture mostly occurs at the original austenite grains. P, S and their compounds precipitate at the grain boundaries, and carbides are at the grain boundaries. The precipitation and accumulation of hydrogen and the accumulation of hydrogen at the grain boundary will increase the sensitivity of the wave gasket to delayed fracture.
1. Examples of broken wave washers
Wave washers (see Figure 1) are stamped and formed by 65Mn steel plate, quenched (810℃×35min), tempered (350℃×60-70min), pickled, surface galvanized, and hydrogen driven (190℃×5h), The hardness is 45-48HRC, and the surface galvanized layer is 5–8μm.
A routine inspection was carried out one month after installation, and it was found that some gaskets had been broken.
2. Test process and results
2. 1 Chemical composition analysis
The chemical composition analysis results of the same batch of raw materials for wave washers are shown in Table 1. It can be seen that the alloying elements and impurity content of the raw materials meet the requirements of the GB/T1222-2007 “Spring Steel” standard.
Table 1 Chemical composition of 65Mn wave washer (mass fraction W%)
Project CMnSP Remarks
Measured value 0.650.980.0190.032
2, 2 Fracture analysis
Check that the galvanized layer on the surface of the three broken wave washers is basically intact without obvious scratches or damage. Observe the macro morphology of the fracture. The cracks all started from the wave trough of the wave washer, and the fractures were fresh and without plastic deformation. The fracture source areas are all located at the place where the gasket is subjected to the greatest force. The cracks originate from the gasket near the surface and are mainly distributed at the grain boundaries, forming intergranular fractures. There are a small amount of shear lip and a small number of dimples in the instantaneous fracture area, which is typical Fracture morphology of hydrogen embrittlement.
2. 3 Organization and hardness inspection
The microstructure of the broken gasket is tempered troostite + a small amount of carbides, and the surface layer has no decarburization.
Measured by the surface Rockwell hardness HR30N, the hardness of the 1# washer is 48.0HRC, and the hardness of the 2# washer is 47.5HRC; the hardness is 520HV5, 525HV5 by Vickers hardness test (the Rockwell hardness is 50.5HRC, 51HRC according to GB1172) .
3. Analysis and verification
3. 1 Analysis of hardness test methods
The broken washer is tested with HV5 and it is 520HV5, 525HV5 (converted into Rockwell hardness of 50.5HRC, 51HRC according to GB1172; the hardness tested by HRA or HR30N is only 47.5-48HRC, indicating that there are obvious differences between the two types of hardness test methods and their results.
The wave washer process regulations stipulate that the hardness shall be measured according to HRA or HR30N, and after consulting the hardness test standard, it is found that the minimum thickness of the HRA testable sample in GB/T230.1-2009 “Metallic Material Rockwell Hardness Test Method” is about 0.56 mm (calculated according to 46.0HRC), the minimum thickness of HR30N test specimens in GB/T1818-1994 “Metallic Material Surface Rockwell Hardness Test Method” is about 0.45mm (calculated according to 46.0HRC), ASTME18-2007 “Metal Materials Rockwell and Surface Rockwell Hardness Test Method “The minimum thickness of testable samples is about 0.63mm (calculated by 46.0HRC), and the minimum thickness of HR30N testable samples is about 0.476 mm (calculated by 46.0HRC). In addition, the minimum thickness of HV5 testable specimen in GB/T4340.1-2009 “Metallic Material Vickers Hardness Test Method” is about 0.2 mm (calculated according to 46.0HRC), and the minimum thickness of HV10 testable specimen is about 0.3 mm (Calculated according to 46.0HRC). The nominal thickness of the wave washer is only 0.45 mm, and the process regulations stipulate that the measurement with HRA or HR30N obviously does not meet the requirements of the hardness test standard. The part thickness is insufficient and the applied load is relatively large, which leads to excessive deformation of the part, which will inevitably make the hardness value tested far lower than the actual hardness of the part. A reasonable hardness test method is to use HV5 or HV10 to determine the hardness of the part.
3. 2 Discussion on the fracture
The wave washer is installed in a flattened state, and the wave crest (or trough) is mainly under tension. The wave washer fractured early, there were no obvious processing defects around the source area, the fracture was flat and showed intergranular fracture, the grain outline was clear, there were torn edges and dimples, no corrosion products and plastic deformation. These characteristics indicate that the fracture of the wave washer is a brittle fracture along the grain. Due to the hardness test method, the actual hardness of the part exceeds the required upper limit, which will increase the brittleness, but a single high hardness will not make the wave washer fracture appear intergranular. It can be seen from the production process of the parts that this intergranular fracture may be the result of the combined effects of hydrogen embrittlement, temper embrittlement and excessive hardness.
3. 3 verification test
In order to identify the relationship between hydrogen embrittlement, temper embrittlement and high hardness, hydrogen embrittlement verification test and temper embrittlement test were carried out respectively.
① Hydrogen embrittlement verification test
Within the scope of the original process parameters (350℃ tempering), after galvanizing the wave washer, the hydrogen drive time was changed, and the hydrogen drive test was performed with 4h, 8h, 12h, and 16h respectively, and the fracture and hydrogen content were checked. Check the fracture of the wave washer that has been driven by hydrogen at different times. The wave washer that was driven by hydrogen at 4h is basically a fracture along the crystal. The wave washer over 8h is a mixed fracture with a small amount of dimples along the crystal, but it has a microscopic fracture. Obvious features along the grain. This may be related to the residual hydrogen and high hardness (the hardness is 52HRC). High hardness means high strength, which increases the hydrogen sensitivity of wave washers and causes hydrogen embrittlement.
②Verification test of temper brittleness
Adjust the tempering temperature to temper at 370℃, 390℃ and 410℃ respectively. After electroplating, check the fracture and Vickers hardness, and perform Rockwell hardness conversion according to GB/T1172. The results are shown in Table 2.
Table 2 Verification test results of temper brittleness
Tempering temperature/℃ Vickers hardness HV5 Rockwell hardness HRC
It can be seen that when tempered at 370℃, the hardness of the wave washer is greater than 51.0HRC, and there are a lot of brittle fractures along the grain + a few dimples. When tempered at 390～410℃, the hardness is 46.5-48.5HRC, with a large number of dimples + individual The ductile fracture along the grain.
When the wave washer with hardness between 46-51HRC is not galvanized, its fracture is ductile, which means that the intergranular fracture of the wave washer is neither the first type of temper brittleness nor the second type of temper brittleness. , Its characteristic changes are mainly related to the hardness level after tempering and the change of hydrogen content.
The chemical composition of the material has no obvious harmful elements. The hydrogen embrittlement fracture is directly related to the residual hydrogen content and strength of the matrix. Using 390-410℃ tempering, the actual hardness of the wave washer can be controlled within the technical requirement of 46-49HRC. The matching of residual hydrogen content after 8 hours of hydrogen drive can ensure that the wave washer does not appear brittle fracture during use.
With the prolongation of hydrogen drive time, the hydrogen content of the wave washer gradually decreases, but the residual hydrogen content basically does not change after 12h of hydrogen drive. Therefore, it is reasonable and effective to use 12h to drive the wave washer. If the hardness is controlled within the range of 43-45HRC Can greatly improve its reliability.
4. Delayed fracture mechanism
Both from the inspection results and theoretical analysis, it is believed that the high hydrogen content is the main reason for the delayed fracture of wave washers. The delayed process is actually the process of hydrogen diffusion and accumulation in the spring steel to cause fracture. There are different opinions on the mechanism of hydrogen-induced cracking. There are three main categories: pressure theory, binding energy theory, and process theory. These three theories have the same description of the hydrogen-induced cracking process. This process can be briefly described as follows:
① The hydrogen in steel comes from two ways: endogenous and external infiltration.
Endogenous refers to the dissolution of hydrogen in steel during smelting. As the temperature decreases, the solubility decreases. Hydrogen precipitates at irregularities in the steel, such as grain boundaries, phase boundaries or microcracks; hydrogen forms interstitial solid solutions in steel. The gap between the austenite is larger than that of ferrite, and the solubility of hydrogen in austenite is greater than that in ferrite. During the cooling process of the steel ingot, hydrogen will inevitably be precipitated when austenite transforms into ferrite.
External penetration means that the wave washer is used in pickling, electroplating or acidic environment, and hydrogen penetrates into the steel from the outside. The increase in hydrogen content of steel from 0.1PPm to 11PPm after pickling and galvanizing of wave washers is a typical example of external hydrogen permeation.
②Below 250℃, the alloying elements and inclusions in the steel can hardly diffuse, and the small hydrogen atom diameter (0.106nm) can still enter and exit actively, so the infiltration and removal of hydrogen is reversible. Even if stored for a long time at room temperature, the infiltrated hydrogen can be slowly released. As the temperature rises, the hydrogen diffusion speed increases, so baking is an effective method of driving hydrogen.
③The uneven structure of steel and the concentration of internal stress will cause hydrogen to accumulate in local areas in the steel. These areas of hydrogen accumulation are commonly known as “hydrogen traps”. The three parameters reflecting the characteristics of traps are: trap density (NX trap), trap depth (UB), and filling degree or concentration (CX). Hydrogen traps can be divided into “tissue traps” and “stress traps”. The delayed fracture of wave washers is the result of the combined effects of tissue traps and stress traps.
④Tissue traps refer to the grain boundaries, phase boundaries, inclusions and matrix junctions and microscopic voids in steel. The hydrogen falling into the traps becomes irreversible hydrogen. The existence of hydrogen traps increases the solubility of hydrogen, reduces the effective diffusion coefficient, increases the local hydrogen solubility, and develops into a crack source. The density and depth of the tissue traps mainly depend on the chemical composition, grain size and microstructure of the steel. Generally speaking, the density and depth of the traps of alloy steel are greater than that of carbon steel; the trap density of martensitic steel is the highest, and the trap density of austenite Steel has the lowest trap density; the sorbite structure has the lowest trap density and the shallowest depth.
⑤ Stress trap refers to the internal lattice distortion, dislocation accumulation, fragmentation of inclusions, microscopic cavities and microcracks caused by steel pressure processing and heat treatment to become the convergence point of hydrogen. The stress trap is a reversible trap, and the hydrogen trap disappears after the stress is removed. The deepest stress trap is often in the area with the greatest tensile stress. The hydrogen dissolved in the steel gradually converges to the deep trap (UB≥58KJ/mol), which further increases the stress at the trap. When the internal stress exceeds the binding force, the Micro-cracks are produced, and as hydrogen continues to diffuse and accumulate, the cracks gradually increase, and eventually the steel material suddenly breaks.
⑥ The depth of the hydrogen trap of the wave washer is related to the stress state of the spring steel during service. The compressive stress of the washer increases, and the depth of the trap increases accordingly. Only when a sufficiently deep trap is filled with a sufficient amount of hydrogen can delayed fracture be induced, and the trap is not deep enough or the degree of hydrogen filling is not enough to cause hydrogen embrittlement, and will not cause delayed fracture. The changes in mechanical properties of steel caused by hydrogen embrittlement and delayed fracture of alloy steel and carbon steel are quite different. Hydrogen embrittlement does not change the tensile strength of steel much, but the plastic index drops sharply, which is concentrated in the reduction of area; delayed The fracture is mainly reflected in the decrease of the tensile strength and impact strength of steel.
⑴Wave washer hardness testing methods have know-how, improper selection, the actual hardness of the wave washer exceeds the technical requirements, resulting in reduced parts toughness and delayed fracture, which is the main cause of brittle fracture of wave washer.
⑵It is better to control the tempering temperature of the wave washer at 400-420℃, choose HV5 to test the hardness, control the hardness at 43-46HRC, and the hydrogen drive time after electroplating is not less than 12h, which can avoid the delayed fracture of the wave washer.
⑶ Wave washers are used to reduce the risk of delayed fracture during pickling and electroplating, and use shot blasting + non-electrolytic dacromet coating for corrosion protection.