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Analysis of the causes and preventive measures for wear of castings in mechanical processing.
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Analysis of the causes and preventive measures for wear of castings in mechanical processing.

Views: 21     Author: Site Editor     Publish Time: 2018-08-24      Origin: www.fuchun-casting.com

Wear is a very common phenomenon in the process of machining, which may be caused by many factors such as workpiece materials, environment, processing methods and so on. Mechanical wear will adversely affect the machining quality, machining accuracy and processing efficiency of products. Therefore, it is very important to do a good job in preventing mechanical wear and tear for friends engaged in the industry of machining.


Common types and characteristics of mechanical wear:

The common wear types in machining include running-in wear, hard particle wear, surface fatigue wear, thermal wear, phase change wear and hydrodynamic wear.

Running-in wear is the wear of machinery under normal load, speed and lubrication conditions. This wear generally develops slowly, and has little effect on machining quality in a short time.

Hard particle wear is caused by the part itself dropping abrasive particles or from the outside into the machine tool hard particles, mixed into the processing area, by mechanical cutting or grinding, causing damage to the parts, which is a more serious impact on the quality of processing.

Surface fatigue wear is a kind of micro-cracks or pits caused by alternating loads, which cause damage to parts. This kind of wear is closely related to the size of pressure, load characteristics, machine material and size.

Hot wear is a phenomenon that the heat produced in the process of friction acts on the parts, making the parts soften by tempering, burn and wrinkle. This kind of wear usually occurs in high-speed and high-pressure sliding friction, wear is more destructive, and accompanied by the nature of accidental wear.

Corrosive wear is a chemical action, that is, chemical corrosion causes wear. When the surface of the part contacts with acid, alkali, saline liquid or harmful gas, it will be eroded by chemistry, or the surface of the part combines with oxygen to form hard and brittle metal oxides which are easy to fall off and cause wear of the part.

Phase change wear is a kind of wear that the parts work at high temperature for a long time. The metal grain on the surface of the parts becomes larger when heated, and the grain boundary is oxidized around, resulting in a small gap, which makes the parts fragile and wear-resistant.

Hydrodynamic wear is the wear on the surface of parts caused by the impact of liquid or particles mixed in the liquid at a faster speed.

Causes and preventive measures of parts wear:

Some wear belong to normal wear category, such as friction between parts, wear caused by hard particles, fatigue wear caused by long-term alternating load, corrosion of chemical substances to parts, and changes of metallographic structure or matching properties of parts surface under high temperature.

For the friction between parts, ensure that the parts have enough cleanliness and lubrication. For wear caused by hard particles, parts need to be exposed to ensure that parts are clean and covered. For the fatigue wear of parts caused by long-term alternating load, clearance can be eliminated and suitable lubricating grease can be selected to reduce the extra vibration of the system and improve the machining accuracy of parts. For the corrosion of chemical substances to parts, it is necessary to remove harmful chemical substances and improve the corrosion resistance of parts. For the change of metallographic structure on the surface of parts or the change of matching properties under high temperature, we should try to improve the working conditions, or adopt high temperature resistant and wear resistant materials to make parts.

Abnormal wear includes wear caused by failure of repair or manufacturing quality to meet design requirements, wear caused by violation of operating procedures and wear caused by improper transportation, loading and unloading, storage. To prevent this kind of wear and tear, we should strictly check the product quality, familiar with mechanical properties, careful operation, and master enough lifting knowledge, be careful in operation, to avoid improper operation.

If the machine is overhauled, the life of the machine may be shortened, which may be due to several reasons, such as deformation of the base parts, damage to the balance of the parts, failure to perform running-in and lower hardness of the parts.

Deformation of basic parts will change the relative position of each part, thus accelerating the wear of parts and shortening the life of parts. Reasonable installation and adjustment can be taken to prevent deformation. Balance failure of parts refers to the unbalanced high-speed rotating parts, accelerating the damage of parts under the action of centrifugal force, shortening the life of parts. Strict action should be taken to balance the test measures. The replacement parts do not perform the running-in of the mating surface will increase the wear amount, so the fitting must be run-in. The selected parts are not properly selected, the surface hardness is not up to the standard, or the heat treatment is not qualified. Materials should be selected according to the requirements and reasonable heat treatment should be carried out.



  • What is 'multiple certification'?

    This is where a batch of steel meets more than one specification or grade. It is a way of allowing melting shops to produce stainless steel more efficiently by restricting the number of different types of steel. The chemical composition and mechanical properties of the steel can meet more than one grade within the same standard or across a number of standards. This also allows stockholders to minimise stock levels.

    For example, it is common for 1.4401 and 1.4404 (316 and 316L) to be dual certified - that is the carbon content is less than 0.030%. Steel certified to both European and US standards is also common.

  • What surface finishes are available on stainless steels?

    There are many different types of surface finish on stainless steel. Some of these originate from the mill but many are applied later during processing, for example polished, brushed, blasted, etched and coloured finishes.

    The importance of surface finish in determining the corrosion resistance of the stainless steel surface cannot be overemphasised. A rough surface finish can effectively lower the corrosion resistance to that of a lower grade of stainless steel.

  • Can I use stainless steel at high temperatures?

    Various types of stainless steel are used across the whole temperature range from ambient to 1100 deg C. The choice of grade depends on several factors:

    1. Maximum temperature of operation
    2. Time at temperature, cyclic nature of process
    3. Type of atmosphere, oxidising , reducing, sulphidising, carburising.
    4. Strength requirement

    In the European standards, a distinction is made between stainless steels and heat-resisting steels. However, this distinction is often blurred and it is useful to consider them as one range of steels.

    Increasing amounts of Chromium and silicon impart greater oxidation resistance. Increasing amounts of Nickel impart greater carburisation resistance.

  • Can I use stainless steel at low temperatures?

    Austenitic stainless steels are extensively used for service down to as low as liquid helium temperature (-269 deg C). This is largely due to the lack of a clearly defined transition from ductile to brittle fracture in impact toughness testing.

    Toughness is measured by impacting a small sample with a swinging hammer. The distance which the hammer swings after impact is a measure of the toughness. The shorter the distance, the tougher the steel as the energy of the hammer is absorbed by the sample. Toughness is measured in Joules (J). Minimum values of toughness are specified for different applications. A value of 40 J is regarded as reasonable for most service conditions.

    Steels with ferritic or martensitic structures show a sudden change from ductile (safe) to brittle (unsafe) fracture over a small temperature difference. Even the best of these steels show this behaviour at temperatures higher than -100 deg C and in many cases only just below zero.

    In contrast austenitic steels only show a gradual fall in the impact toughness value and are still well above 100 J at -196 deg C.

    Another factor in affecting the choice of steel at low temperature is the ability to resist transformation from austenite to martensite. 

  • Is stainless steel non-magnetic?

    It is commonly stated that “stainless steel is non-magnetic”. This is not strictly true and the real situation is rather more complicated. The degree of magnetic response or magnetic permeability is derived from the microstructure of the steel. A totally non-magnetic material has a relative magnetic permeability of 1. Austenitic structures are totally non-magnetic and so a 100% austenitic stainless steel would have a permeability of 1. In practice this is not achieved. There is always a small amount of ferrite and/or martensite in the steel and so permeability values are always above 1. Typical values for standard austenitic stainless steels can be in the order of 1.05 – 1.1. 

    It is possible for the magnetic permeability of austenitic steels to be changed during processing. For example, cold work and welding are liable to increase the amount of martensite and ferrite respectively in the steel. A familiar example is in a stainless steel sink where the flat drainer has little magnetic response whereas the pressed bowl has a higher response due to the formation of martensite particularly in the corners.

    In practical terms, austenitic stainless steels are used for “non-magnetic” applications, for example magnetic resonance imaging (MRI). In these cases, it is often necessary to agree a maximum magnetic permeability between customer and supplier. It can be as low as 1.004.

    Martensitic, ferritic, duplex and precipitation hardening steels are magnetic.

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