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What are the causes of casting stress? Are there any solutions?
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What are the causes of casting stress? Are there any solutions?

Views: 49     Author: Site Editor     Publish Time: 2018-07-11      Origin: www.fuchun-casting.com

Factors affecting casting stress

During solidification and cooling, the total stress of the casting is the algebraic sum of thermal stress, phase change stress and mechanical hindrance stress.

Thermal stress and phase transformation stress are often the residual stresses in castings. Residual stress is mainly related to the following four factors:

1. Metal properties

The residual stress of metal is directly proportional to the elastic modulus of metals. For example, the residual stress of metals with large elastic modulus such as cast steel and white iron is larger than that of gray cast iron with small elastic modulus, which confirms this theory.

2. The residual stress of the casting is proportional to the free line contraction coefficient of the alloy used.

The thermal conductivity of the metal will affect the temperature difference between the inside and outside of the casting. The thermal conductivity of carbon steel is higher than that of carbon steel, so the casting stress of carbon steel will be less than that of alloy steel  under the same standard.

2. Mold properties;

The heat storage coefficient represents the cooling rate of the casting, and the cooling of the casting will cause the temperature difference between inside and outside to increase. Large temperature difference will lead to greater casting stress. Therefore, the stress of metal castings is greater than that of sand castings.

3. Casting condition

Increasing the pouring temperature makes the temperature distribution more uniform after pouring, and also makes the cooling rate of the castings quicken, which can eliminate the casting stress.

Positive influence.

4. Casting structure

The greater the difference between the inner and outer wall thickness of a casting, the greater the difference between the cooling rate of the surface during casting, and the greater the casting stress.


    stress-strain curves of die cast alloys

Measures to reduce and eliminate casting stress

1. Rationally design the structure of the casting.

In the process of casting design, the wall thickness should be made as uniform as possible, and the phenomenon of protruding sharp angle should be avoided. This design method can effectively reduce the production of casting stress. In order to solve the problem of casting deformation, the following measures are put forward:

(1) When designing the castings, the inner and outer wall thickness of the castings should be basically the same, and the inner and outer walls of the castings should be cooled together with the castings as far as possible.

(2) To make the overall structure of the castings symmetrical in the design of the castings, even if the internal stress exists to offset the stress in the opposite direction

(3)  Reverse denaturation.

 When casting the shape of the castings with the same casting stress, condensation is to take some measures to make the internal and external cooling rate of the castings basically the same. The aim is to make the temperature difference between inside and outside of the casting small, so as to avoid the deformation of the casting. Therefore, the casting gate can be set skillfully in the casting process.

2. Adopt simultaneous solidification process

At the same time, condensation means taking some measures to make the casting inside and outside the cooling rate basically the same. The aim is to make the temperature difference between inside and outside of the casting small, so as to avoid the deformation of the casting. Therefore, the casting gate can be set skillfully in the casting process.

3. Aging treatment

3.1 Artificial aging

Temperature and retention time for different metal materials in eliminating residual stresses should be determined according to specific conditions. In the casting process, the usual method is heating to the elastic-plastic state, and cooling for a period of time to eliminate the stress.

In order to make the temperature distribution uniform in the process of heating and cooling, the temperature control should be carried out uniformly. In the actual production process for the casting cooling or heating speed and retention time should be set according to specific circumstances.

3.2 Natural aging

Natural aging refers to the casting after casting in the open air environment for a long time, casting stress will naturally disappear over time. The residual casting stress will cause the lattice deformation of the casting and the internal instability. This method is the simplest and does not require any cost. But in modern society where time is scarce, this primitive method is seldom adopted.



  • 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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