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Some basic knowledge about Stainless Steel
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Some basic knowledge about Stainless Steel

Views: 75     Author: Site Editor     Publish Time: 2018-01-23      Origin: Site

What is stainless steel

Stainless steel is the abbreviation of rust resistant and acid resistant steel. It is called stainless steel with weak corrosion medium, such as air, steam and water, or stainless steel with rust, and the steel resistant to chemical corrosion medium (acid, alkali, salt and other chemical etching) is called acid resistant steel.


In practical applications, because of their differences in chemical composition, the former is not necessarily resistant to chemical corrosion, while the latter is generally non corrosive. The corrosion resistance of stainless steel depends on the alloy elements contained in the steel.

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Common classification:

Usually, according to metallographic structure, ordinary stainless steel can be divided into three types: austenitic stainless steel, ferritic stainless steel and martensitic stainless steel.


On the basis of these three basic metallographic structures, for specific needs and purposes, the double phase steel, precipitated hardened stainless steel and high alloy steel with less than 50% iron content have been derived.


1. Austenitic stainless steel.

A stainless steel based on the austenitic structure (CY phase) of a surface centered cubic crystal, which is not magnetic and is mainly reinforced by cold machining (and may lead to a certain magnetism). The American Steel Association marked 200 and 300 series numbers, such as 304.


2. Ferritic stainless steel.

The matrix is mainly composed of ferrite (a phase) in the body centered cubic crystal structure, which is magnetic, and can not be hardened by heat treatment, but cold processing can make the stainless steel slightly strengthened. The American Steel Association is marked by 430 and 446.


3. Martensitic stainless steel.

The matrix is martensite structure (body centered cubic or cubic), and has magnetic properties, which can adjust the mechanical properties of stainless steel by heat treatment. The American Steel Association marked 410, 420 and 440. Martensite has austenite structure at high temperature, and when austenite is cooled at room temperature, the austenite can be transformed into martensite.


4. AUSTENITIC FERRITIC (biphasic) stainless steel.

The matrix has both austenite and ferrite two phase structure, in which the content of the less phase matrix is generally more than 15%, it is magnetic, and the stainless steel which is strengthened by cold processing and 329 is the typical double phase stainless steel. Compared with austenitic stainless steel, dual phase steel has high strength, good resistance to intergranular corrosion, chloride stress corrosion and pitting corrosion.


5. Precipitated hardened stainless steel.

The matrix is austenite or martensite and can be hardened by precipitation hardening. The American Steel Association has 600 series of digital markup, such as 630, 17-4PH.Generally speaking, the corrosion resistance of austenitic stainless steel is excellent except for alloy. Ferritic stainless steel can be used in low corrosive environment. In mild corrosive environment, martensitic stainless steel and precipitated hardened stainless steel can be used in the material with high strength or high hardness.


How to deal with rust stains on stainless steel?

1. Chemical methods

The rust area is repassivated by pickling paste or spray to form chromium oxide film to restore its corrosion resistance.After pickling, in order to remove all pollutants and acid residues, it is very important to rinse properly with clean water. After all the treatments are polished with polishing equipment and sealed with polished wax. For local slight rust, you can also use 1:1 gasoline and oil mixture to wipe rust spots with clean rag.


2. Mechanical method

The methods of cleaning rust spots are: sand blasting, cleaning with glass or ceramic particles, annihilation, scrubbing and polishing. By mechanical means, it is possible to wipe off the pollution caused by previously removed materials, polishing materials or annihilation materials.


All kinds of pollution, especially foreign iron particles, may be the source of corrosion, especially in humid environments. Therefore, the mechanical cleaning surface should preferably be cleaned under dry conditions. The mechanical method can only clean up the surface, and can not change the corrosion resistance of the material itself.It is suggested that after polishing the machine, the polishing equipment should be re polished and polished.

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                                      (a)                                                                              (b)                                                                                                             (c)                    




  • 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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