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What is casting stress? What are the effects of stress?
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What is casting stress? What are the effects of stress?

Views: 64     Author: Site Editor     Publish Time: 2018-04-11      Origin: www.fuchun-casting.com


When a casting solidifies, with the decrease of temperature, the casting will become solid in this process will occur phase transformation, as the casting into solid phase transformation process, phase transformation will also occur shrinkage or expansion process, due to the existence of a thicker outer wall of the casting, the temperature of the outer layer drops rapidly, but the temperature of the inner layer will drop very slowly. Therefore, the thicker the wall thickness is, the longer the cooling time will be.


This phenomenon results in different shrinkage of the inner and outer layers of a complete castings and different thicknesses. This phenomenon will affect the mutual restraint between the inner and outer layers of the castings, resulting in tensile or irreversible deformation of the castings. Thermal stress is a phenomenon that the thickness of the casting wall is not uniform in the same unit time and the degree of shrinkage is different.


These stresses can lead to fracture or plastic deformation depending on the other variables of heating, which include material types and constraints. Temperature gradients, thermal expansion or contraction and thermal shocks are things that can lead to thermal stress. This type of stress is highly dependent on the thermal expansion coefficient which varies from material to material. In general the larger the temperature change, the higher the level of stress that can occur.


Stress induced deformation


Effects of casting stress:

The remnants of casting stress have a very bad effect on the quality of a casting.Residual stress is an important factor that causes deformation during the production and storage of castings. It will reduce some properties of castings after long-term accumulation.


For example, the direction of residual stress obtained by a casting and its direction of working stress are duplicated, then in this casting working time, the two forces will produce mutual effect, resulting in the direction of the force beyond the scope of the casting material, so that the casting deformation and even fracture.And in large-scale production process, if the castings are deformed, the castings can not be put into the pre-prepared mold, which leads to the failure of production line.In addition, for some castings with high precision requirements,if the residual stress in the castings causes the deformation of the castings, then it will greatly affect the accuracy, but also cause economic losses.


Reference: en.wikipedia.org/wiki/Residual_stress

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  • 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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