Practical dry goods! Summary of failure analysis of metal materials and parts

Date：2021-12-21 15:46:15 Views：1933

The failure forms and causes of metal materials are closely related. The failure forms are the apparent characteristics of the material failure process, which can be observed in an appropriate way. The failure reason is the physical and chemical mechanism leading to component failure, which needs to be diagnosed and demonstrated through the investigation and research of failure process and the macro and micro analysis of failed parts.

Common methods for failure analysis of metal materials:

Fracture analysis, metallographic structure analysis, composition analysis, phase analysis, residual stress analysis, mechanical property analysis (hardness, tensile property, impact property, bending property, hardness, etc.), investigation and verification of on-site process and service environment, etc.

Significance of failure analysis of metal materials:

1. Failure analysis can reduce and prevent the occurrence of similar failure phenomena, so as to reduce economic losses and improve product quality.

2. Provide information for enterprise technology development and technological transformation, increase the technical content of enterprise products, and obtain greater economic benefits.

3. Analyze the failure causes of mechanical parts to provide scientific basis for accident liability identification, detection of criminal cases, determination of compensation liability, insurance business, modification of product quality standards, etc.

Classification of failure forms of metal materials and parts:

Elastic deformation failure: elastic deformation failure occurs when the recoverable elastic deformation of the material caused by stress or temperature is large enough to affect the normal performance of the predetermined function of the equipment.

Plastic deformation failure: plastic deformation failure occurs when the loaded material produces unrecoverable plastic deformation large enough to affect the normal function of the equipment.

Ductile fracture failure: the fracture in which a material produces significant macro plastic deformation before fracture is called ductile fracture failure.

Brittle fracture failure: the fracture in which the material has no or little macroscopic plastic deformation before fracture is called brittle fracture failure.

Fatigue fracture failure: the fracture of materials under alternating load after a certain period is called fatigue fracture failure.

Corrosion failure: corrosion is the physical or chemical reaction between the material surface and the service environment, resulting in damage or deterioration of the material. If the corrosion of the material makes it unable to play its normal function, it is called corrosion failure. There are many forms of corrosion, including uniform corrosion all over the material surface and local corrosion only in local places. Local corrosion is divided into point corrosion, intergranular corrosion, crevice corrosion, stress corrosion cracking, corrosion fatigue, etc.

Wear failure: when the materials are in contact with each other or the material surface is in contact with the fluid and makes relative movement, the shape, size or quality of the material surface changes due to physical and chemical effects, which is called wear. The loss of component function caused by wear is called wear failure. There are many forms of wear, including adhesive wear, abrasive wear, impact wear, fretting wear, corrosion wear, fatigue wear and so on.

Common types of metal material failure:

Failure caused by improper design (unreasonable structural design, insufficient design hardness, improper material selection and unreasonable material state requirements);

Failure caused by material defects (porosity, segregation, subcutaneous bubbles, shrinkage cavities, non-metallic inclusions, white spots, non-metallic inclusions, surface corrosion, etc.);

Failure caused by casting defects (shrinkage cavity and porosity, white mouth and anti white mouth, poor spheroidization of nodular cast iron, slag inclusion, segregation carbide, casting crack, graphite floating, etc.);

Failure caused by forging defects (overheating and overburning, forging cracks, hot embrittlement and copper embrittlement, forging folding, high temperature oxidation, insufficient annealing, forging white spots, forging streamline defects, etc.);

Failure caused by welding defects (welding crack, incomplete penetration and fusion, improper welding preheating, slag inclusion and porosity, intergranular corrosion and stress corrosion);

Failure caused by heat treatment defects (quenching cracks, surface decarburization, carburizing / nitriding defects, tempering cracks, etc.);

Failure caused by cold forming defects (grinding defects, cutting defects, cold heading defects, punching / extrusion / tensile forming defects, etc.).

Common defects causing failure

Microstructure defects of as cast metal

The common structural defects of as cast metal include shrinkage cavity, porosity, eccentricity, internal crack, bubble and white spot.

Shrinkage cavity

During the condensation process, the metal forms tubular (or horn shaped) or dispersed holes in the ingot or casting center due to the shrinkage of volume, which is called shrinkage cavity. The relative volume of shrinkage cavity is related to the temperature of liquid metal, cooling conditions and the size of casting. The higher the temperature of liquid metal, the greater the volume difference between liquid and solid, and the larger the volume of shrinkage cavity. When pouring metal into the thin-walled mold, the thinner the mold wall, the faster the heating, and the harder it is for the liquid metal to cool. When the mold is just poured, the larger the volume of the liquid metal and the larger the shrinkage cavity after metal condensation.

loose

Pouring metal under the condition of rapid cooling can avoid the formation of concentrated shrinkage cavities in the upper part of the ingot, but at this time, the volume difference between liquid metal and solid metal still maintains a certain value. Although large shrinkage cavities seem to have been eliminated on the surface, many small shrinkage cavities, i.e. loose, are distributed in the whole volume of the metal.

In the process of forging and rolling, the looseness of steel can be greatly improved, but if the looseness of the original ingot is serious and the compression ratio is insufficient, the more serious looseness will still exist after hot working. In addition, when there are many bubbles in the original ingot, and the welding is poor in the hot rolling process, or the bubble distribution in the boiling steel is poor, which affects the welding and may also form porosity.

The existence of porosity has great harm, mainly including the following: (1) in castings, due to the existence of porosity, its mechanical properties are significantly reduced, which may become a fatigue source and fracture in the process of use. In the castings used as liquid containers or pipes, sometimes there are basically interconnected looseness, so that they can not pass the hydrostatic test, or leakage occurs in the process of use; (2) If there is porosity in steel, its mechanical properties will also be reduced. However, porosity can generally be reduced or eliminated in the process of hot working, so the effect of porosity on the properties of steel is smaller than that of castings; (3) There is serious porosity in the metal, which has a certain impact on the surface roughness after machining.

segregation

In the process of metal condensation, the phenomenon of uneven chemical composition formed due to the influence of some factors is called segregation. Segregation can be divided into intragranular segregation, intergranular segregation, regional segregation and specific gravity segregation.

Due to insufficient diffusion, there is a phenomenon of uneven composition in the crystal range in the solidified metal, that is, intracrystalline segregation. For the same reason, in solid solution metals, the composition of post solidified crystals and pre solidified crystals will also be different, that is, intergranular segregation. Carbide segregation is a kind of intergranular segregation.

When pouring the casting key (or casting), due to the strong directional heat dissipation through the mold wall, a large temperature difference will be formed in the solidified alloy. The result will inevitably lead to the enrichment of high melting point components in the outer region and low melting point components in the heart, as well as non-metallic impurities and gases precipitated during solidification. This segregation is called Regional segregation.

In the process of metal condensation, if the density of the precipitated crystals is different from that of the remaining solution, these crystals tend to sink or float in the solution, and the phenomenon of uneven chemical composition is called specific gravity segregation. The greater the density difference between the crystal and the remaining solution, the greater the specific gravity segregation. This density difference depends on the density difference of metal components and the composition difference between crystal and solution. If the cooling is slower and the increase of the number of primary crystals is slower with the decrease of temperature, the larger the temperature range in which the crystals can float and sink freely in the solution, and the stronger the specific gravity segregation.

Bubble

Metal can dissolve a large amount of gas in the molten state. In the condensation process, the solubility decreases sharply with the decrease of temperature, resulting in the release of gas from liquid metal. If the metal has completely solidified at this time, the remaining gas is not easy to escape, and some of it is contained in the metal still in the plastic state, thus forming pores, which are called bubbles.

The harmful effects of bubbles are as follows: (1) bubbles reduce the effective section of metal castings, and greatly reduce the strength of materials due to their notch effect; (2) When there are bubbles on the ingot surface, they may be oxidized during hot forging and heating, and can not be welded in the subsequent forging process to form fine lines or cracks; (3) In boiling steel and some alloys, segregation may also occur due to the existence of bubbles, resulting in cracks.

White dot

On the cross section after erosion, there are many short and discontinuous hairy cracks; On the longitudinal section, round or oval spots with smooth surface and silver white will be found. This defect is called white spot.

White spots are most likely to occur in alloy structural steels and low alloy tool steels with nickel, chromium and manganese as alloy elements. White spots have never been found in austenitic steel and ledeburite steel; White spots may also be found in cast steel, but they are very rare; White spots occasionally occur in the fusion weld metal of the welded workpiece. The generation of white spots is also related to the size of steel. Steel with cross-section diameter or thickness less than 30mm is not easy to produce white spots.

Generally, the longitudinal tensile strength and elastic limit of steel with white spots do not decrease much, but the elongation decreases significantly, especially the reduction of area and impact toughness, which may be close to zero sometimes. The transverse mechanical properties of this steel are much lower than the longitudinal mechanical properties. Therefore, steel with white spots cannot be used in general.

Defects of metal forging and rolling parts

Coarse widmanstatten tissue

When the hot rolling or stop forging temperature is high, due to the coarse austenite grain, the first precipitates precipitated along the grain boundary during subsequent cooling, and grew into the grain in a certain direction, or arranged in parallel or at a certain angle. This morphology is called widmanstatten structure. The precipitates are related to the composition of steel. Hypoeutectoid steel is ferrite and hypereutectoid steel is cementite. The widmanstatten structure increases the brittleness and decreases the strength of the material because of its coarse structure. More important workpieces are not allowed to have widmanstatten structure.

Network carbide and banded structure

For tool steel, the purpose of forging and rolling is not only to form the blank, but also to make the carbide in it broken and evenly distributed.

Decarburization of steel surface

When the steel is heated, the carbon atoms on the metal surface are burned, so that the carbon composition of the metal surface is lower than that of the inner layer. This field is called decarburization, and the surface layer after reducing the carbon content is called decarburization layer. The hardness and strength of the decarburized layer are low, and it is easy to crack under stress and become the crack source. Most parts, especially those requiring high strength and subjected to bending force, should avoid decarburization layer. Therefore, the forged and rolled steel parts shall be arranged for cutting to remove the decarburization layer.

fold

Folding is usually formed because the sharp corners or ears of the material surface produced in the previous forging and rolling are pressed into the metal itself in the subsequent forging and rolling. The folding of steel surface can be removed by machining.

scratch

In the process of production and transportation, the groove mark formed by mechanical scratch on the steel surface is called scratch, also known as scratch or scratch. The existence of scratch defects can reduce the strength of metal; In addition to reducing the strength of steel sheet, it will also cause stress concentration and fracture like notch; Especially during pressing, it will become the center of crack or crack propagation. For pressure vessels, serious scratches are not allowed on the surface, otherwise it will become the starting point of accidents during use.

Spot scar

The surface of metal ingots and profiles is often rough and uneven due to improper treatment. These pits are not deep, generally only 2 ~ 3mm. Because of its irregular shape and different sizes, this rough pit is called scar, also known as spot scar.

If the scab exists on the plate, especially on the main plate, it can not only become the center of plate corrosion, but also produce cracks during punching. In addition, the steel used for manufacturing springs and other parts is not allowed to have scar defects. Because scarring is easy to cause stress concentration, leading to fatigue cracks, which greatly affects the life and safety of the spring.

surface crack

The network crack or notch on the steel surface is caused by high sulfur and low manganese in the steel, or by too high copper content and too many non-metallic inclusions in the steel.

layered

Due to non-metallic inclusions, internal cracks not welded, residual shrinkage cavities, pores and other reasons, the cross section of the sheared steel presents black lines or black bands, and the steel is separated into two or more layers, which is called delamination.

Inclusions and their effects on steel properties

(1) Classification of inclusions

During the processing deformation of steel, various inclusions have different deformability, which can be divided into three categories according to their deformation capacity:

Brittle inclusion

Generally, it refers to simple oxides (Al2O3, Cr2O3, ZrO2, etc.) without plastic deformation ability, double oxides (such as FeO · Al2O3, MgO · Al2O3, Cao · 6 Al2O3), carbides (TIC), nitrides (tin, Ti (CN) AlN, VN, etc.) and undeformed spherical or point inclusions (such as spherical calcium aluminate and silicate with high SiO2, etc.).

Aluminum silicon calcium inclusions in steel have high melting point and hardness. When the deformation of pressure machining increases, aluminum silicon calcium is crushed and distributed in a chain along the machining direction, which seriously destroys the uniform continuity of steel matrix.

Plastic inclusion

This kind of inclusion has good plasticity when the steel is subjected to processing deformation and extends into a strip along the rheological direction of the steel. It belongs to this kind of inclusion, such as ferromanganese silicate with low SiO2 content, manganese sulfide (MNS), (Fe, Mn) s, etc. The interface between inclusions and steel matrix is well bonded, and the tendency of cracks is small.

Inclusion of semi plastic deformation

It generally refers to various composite aluminosilicate inclusions. The matrix in the composite inclusions produces plastic deformation during hot working deformation, but the inclusions distributed in the matrix (such as Cao · Al2O3, spinel double oxide, etc.) do not deform. The matrix inclusions extend with the deformation of the steel matrix, while the brittle inclusions do not deform and still maintain the original geometry, Therefore, it will hinder the free extension of adjacent plastic inclusions, and the part away from brittle inclusions will extend freely along the deformation direction of steel matrix.

(2) Effect of inclusions on properties of steel

A large number of test facts show that inclusions have little effect on the strength of steel and do great harm to the toughness of steel, and the harm degree increases with the increase of steel strength.

Effect of inclusion deformability on properties of steel

The relationship between the deformation behavior of non-metallic inclusions in steel and steel base can be expressed by the relative deformation between inclusions and steel matrix, that is, the deformation rate of inclusions V, and the deformation rate of inclusions can be changed in the range of V = 0 ~ 1. If the deformation rate is low, the steel will be deformed after processing. Due to the plastic deformation of steel and the basic non deformation of inclusions, stress concentration occurs at the junction of inclusions and steel matrix, resulting in microcracks at the junction of steel and inclusions. These microcracks become the hidden danger of fatigue failure during the use of parts.

Stress concentration caused by inclusions

The smaller the thermal expansion coefficient of the inclusion, the greater the tensile stress and the greater the harm to the steel. When processing deformation at high temperature, the difference of thermal shrinkage between inclusions and steel matrix causes cracks at the interface. It is likely to become a potential fatigue failure source in the matrix. The most harmful inclusions are foreign oxides from slag and refractory materials.

Inclusions and toughness of steel

The content of MNS inclusions in ultra-high strength steel and carbon steel has no obvious effect on strength, but can reduce toughness. The fracture toughness decreases with the increase of sulfur content, which has obvious regularity.

From the comparison of inclusion types, the effect of sulfide on toughness is greater than that of nitride. In nitride, ZrN is less harmful to toughness. When the inclusion types are different and the content is similar, the effect of deformed long strip MNS on fracture toughness is greater than that of undeformed sulfide (ti-s, zr-s).

Tandem or spherical sulfide pair ψ And a kV are unfavorable. In terms of the harm to the short transverse sample, the harm of string is more serious than that of ball.

The above is the relevant content of "failure analysis of metal materials and parts" brought by the core test. Through this paper, I hope it can be helpful to you. If you like this article, you might as well continue to pay attention to our website, and we will bring more wonderful content later. If you have any needs related to the inspection and testing of electronic products, please call Chuangxin testing, and we will serve you wholeheartedly.