The failure form and the cause of failure of metallic materials are closely related. The failure form is the apparent characteristics of the material failure process and can be observed by appropriate means. The cause of failure is the physicochemical mechanism leading to the failure of the component, which needs to be diagnosed and demonstrated through failure process research studies and macroscopic and microscopic analysis of the failed component.
The failure modes of materials in various engineering applications mainly consist of fracture, corrosion, wear and deformation, among which fracture failure is the most hazardous.
Elastic deformation failure: Elastic deformation failure occurs when the stress or temperature causes recoverable elastic deformation of the material large enough to affect the normal performance of the intended function of the equipment.
Plastic deformation failure: Plastic deformation failure occurs when the material under load produces irrecoverable plastic deformation large enough to affect the normal performance of the intended function of the equipment.
Toughness fracture failure: The fracture of a material that produces significant macroscopic plastic deformation prior to fracture is called toughness fracture failure.
Brittle fracture failure: Material fracture without or little macroscopically visible plastic deformation before fracture is called brittle fracture failure.
Fatigue fracture failure: Material under alternating load, after a certain period of fracture is called fatigue fracture failure.
Corrosion failure: corrosion is the surface of the material and the service environment physical or chemical reaction, so that the material damage or deterioration of the phenomenon, the corrosion of the material so that it can not play a normal function is called corrosion failure. Corrosion has a variety of forms, there is uniform throughout the material surface uniform corrosion and only in local places where the local corrosion, local corrosion is divided into pitting corrosion, intergranular corrosion, crevice corrosion, stress corrosion cracking, corrosion fatigue, etc.
Wear loss: When materials are in contact with each other or material surfaces are in contact with fluids and in relative motion, the process of changing the shape, size or quality of the material surface due to physical and chemical effects is called wear. The loss of function of a component due to wear is called wear and tear. There are various forms of wear, among which adhesive wear, abrasive wear, impact wear, micro-motion wear, corrosion wear, fatigue wear, etc. are common.
Analysis of the causes of failure
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1. Improper design
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2. Improper selection of materials and material defects
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3. Unreasonable manufacturing process
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4. Improper use of operation and maintenance
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5. Others
Inclusions and their effect on steel properties
(1) Classification of inclusions
Steel in the processing of deformation, all types of inclusions deformability is different, according to its deformation capacity is divided into three categories:
Brittle inclusions
Generally refers to those simple oxides (Al2O3, Cr2O3, ZrO2, etc.), double oxides (such as FeO-Al2O3, MgO-Al2O3, CaO-6 Al2O3), carbides (TiC), nitrides (TiN, Ti(CN)AlN, VN, etc.) and non-deformable spherical or point inclusions (such as spherical calcium aluminate and silicates with high SiO2 content, etc.).
Plastic inclusions
These inclusions have good plasticity when the steel is subjected to processing deformation, along the rheological direction of the steel extends into a strip, belonging to this type of inclusions containing a low amount of SiO2 iron manganese silicate, manganese sulfide (MnS), (Fe, Mn)S, etc.. Inclusions and the steel matrix at the interface between the combination is very good, the tendency to produce cracks is small.
Semi-plastic deformation of inclusions
Generally refers to a variety of composite aluminosilicate inclusions, composite inclusions in the matrix, plastic deformation in the process of thermal processing deformation, but the distribution of inclusions in the matrix (such as CaO-Al2O3, spinel-type double oxide, etc.) is not deformed, the matrix inclusions with the deformation of the steel matrix and extension, while brittle inclusions do not deform, still maintain the original geometry, and therefore will prevent the adjacent plastic inclusions. Therefore, the adjacent plastic inclusions will be prevented from extending freely, while the part away from the brittle inclusions along the steel matrix deformation direction of free extension.
(2) The impact of inclusions on steel properties
A large number of test facts show that the inclusions on the strength of steel has a small impact, on the toughness of steel is more harmful, and the degree of harm with the increase in the strength of steel and increase.
Inclusions and the toughness of steel
The content of MnS inclusions in ultra-high strength steels and carbon steels has no significant effect on strength, but can make the toughness decrease. Among them, fracture toughness decreases with increasing sulfur content, with obvious regularity.
From the inclusions type comparison, sulfide influence on toughness is greater than nitride, in nitride ZrN less harmful to the toughness, inclusions type different and similar content of the case, transformed into a long strip of MnS on the fracture toughness is greater than the non-deformation of sulfide (Ti-S, Zr-S).
Tissue defects arising from heat treatment of metals
Metal heat treatment defects refer to the heat treatment production process generated in the parts to lose the value of use or do not meet the requirements of the technical conditions of the various subsidies, as well as the heat treatment after the heat treatment of the subsequent process performance deterioration or reduce the use of the performance of the heat treatment hidden danger.
The most dangerous defects are cracks, the most important of which are quenching cracks, followed by heating cracks, delayed cracks, cold treatment cracks, tempering cracks, aging cracks, grinding cracks and plating cracks.
Causes of quenching cracks:
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(1) Raw material defects (metallurgical defects extended into quenching cracks)
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(2) The original organization is poor (such as coarse organization in steel or Weiss organization tendency is large)
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(3) Inclusions
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(4) Improper quenching temperature
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(5) Improper cooling during quenching
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(6) Machining defects
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(7) Not timely tempering
The most common defects are deformations, of which quenching deformations account for the majority, and arise from phase changes and thermal stresses. Residual stresses, tissue failure, property failure, brittleness and other defects occur with less frequency and severity.
There are two sources of internal stress:
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(1) Cooling process parts surface and the center of the cooling rate is different, the volume contraction in the surface and the center is not the same. This difference in volume shrinkage due to temperature differences caused by the internal force is called "thermal stress".
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(2) Steel parts in the organizational transformation than the volume change, such as austenite transformation into martensite than the volume increase. Due to the transformation of each part of the cross-section of the successive different, its volume changes in different places, resulting in the amount of internal stress called "tissue stress".
