Function of some groups of stainless steel

The function of some groups of stainless steel can be classified to 5 groups: Martensite stainless steel, Ferrite stainless steel, Austenite stainless steel, Austenite-Ferrite stainless steel, Austenite - Martensite stainless steel

1. Martensite stainless steel:

This stainless steel group contains from 12% to 17% Cr. If the Cr content is at the lower limit (12 ~ 13%) then we have to control the carbon content C is not exceed 0.4% to avoid creating too much chromium carbide. Because Chromium Carbide depleted Cr content in the alloy background and reduce corrosion resitance. If the Cr content is up to 17%, the amount of C can be as high as 0.9 ~ 1.1% to increase the mechanical properties (stiffness) and still remain corrosion resistance.
Martensitic stainless steel ball SALUC
Martensitic stainless steel ball SALUC 

The suitable temperature for heat treatment of this steels is: 950 ~ 1100 ° C. The coolling environment may be oil or air (due to of high content of % Cr so it should be easy to fabricate). The Ram temperatures depends on the specific requirements but must pay attention to avoid brittle ram type II in the temperature of 350 ~ 575 ° C (by quenching in oil. If stainless steel was cooled slowly will forms Cr23C6 steel brittle and reduce the corrosion resistance).

Martensite stainless steel has a high corrosion resistance in freshwater environments, because of the passive effect of chromium so that it is not corrode in acidic HNO3 (but corrosive in other acid). This stainless steels with low carbon content (eg: grade 403 and 410 of U.S) is used for stainless steel jewelry, stainless fasteners, heating equipments (<450 ° C), eg: steam turbine wings .. The stainless steel grade has % of C higher (ex: 420), hardness and higher elastic limit is used for making spring stainless steel, measuring instruments ... The stainless steel grade has % of C up to 0.9 ~ 1.0% (ex: 440, 440B) are used for making surgical instruments (knives, scissors), heat-resistant and abrasion-resistant exhaust valve, some of bearings works in corrosive environments.

2. Ferrite stainless steel:

Ferrite stainless steel has a higher elastic limit than Austenite stainless steel but the plastic deformation is lower, so, they are suitable for the normal fabrication (rolling, drawing, typing , stamping, etc.). Corrosion resistance of them depend on chromium content. In order to limit the local corrosion (point corrosion), we have to increase the chromium content above 20% and better increase more 2% of Mo alloy to be used in the marine enviroment, seawater and acid.
Ferrite Stainless Steel 410 304 Stainless Steel Composite
Ferrite Stainless Steel 410 304 Stainless Steel Composite

Depending on the Cr content, ferrite stainless steel be divided into three groups:
  • The stainless steel group 1 contains about 13% Cr (eg: 405), has very little of carbon (<0.08%). Add ~ 0.2% Al will prevent the forming of austenitic when ignition and facilitate for welding. Group of this stainless steel is widely used in the oil industry.
  • The stainless steel group 2 contains up to 17% of Cr (eg: 430) is a Ferrite stainless steel group and it is most widely using because they can replace the austenite stainless steel in some allow conditions. This group is not contain Ni so it should be much cheaper. This stainless steel group is used widely in HNO3 acid production industry, the food processing, the architecture decoration ... The main disadvantage of this group is difficult to weld, when the temperature exceeds to 950 ° C, the area near the weld becomes brittle and be corroded at the connection (This problem can be overcome by lower % C or adding more content of Ti into stainless steel).
  • The stainless steel group 3 contains Cr from 20 ~ 30% Cr (ex: 446, 446B) have a very high antioxidant properties (desquamating not at 800 ~ 900 ° C).

3. Austenite stainless steel:

Austenite stainless steel is stainless steel group which has an organized austenite even at a temperatures lower than room temperature (~25C) by increasing the Ni content in the stainless steel to the appropriate level. Typical for this steel grade 304 (18-8) and 316 (18 - 10).
The ferritic stainless steel
The ferritic stainless steel on the left has a body centered cubic

The main advantages of this steel group:

  • High corrosion resistance: Completely stable in fresh water, salt water, saturated steam water, in over-heated, in a salt liquid. In any acidic enviroment of HNO3 acid, in normal H2SO4 acid, and in the dilute hydrochloric acid. Therefore, they (Austenite stainless steel) are used in industry of acid production, petro-chemical, food processing and mechanical details which heat resistant up to 900 ~ 1000 ° C.

Austenitic stainless steel pipe Image
Austenitic stainless steel pipe Image
  • High ductility (strain is about from 40 ~ 60%) so it is easy rolling, stamping in normal fabrication. It is  suitable to produce all kinds of bottle and stainless steel tube. They also have a network centered cubic structure so it does not get brittle even when in large particles (due to heating). Especially they have no changes plasticity deformation and brittle in fabrication. Therefore, they can be used at very low temperatures such as in ice land, liquefied gas cylinders and cooling techniques.
  • Mechanical characteristic is ensure: Although this group can not be hardened by heat treatment method (because there is no phase shift) but strong hardening by cold plastic deformation. The main reason is most of austenite at deformation area was transformed into martensite (called martensite deformation). For this reason, stainless steel turn to hard very quickly after each deformation. In order to be able to continue the deformation they need to be incubated at the appropriate temperature.

The main disadvantages of Austenite stainless steel:

  • Expensive: This stainless steel group contains many Ni. But we can can reduce costs by using Mn substitute for Ni in the steel grade 201, 202 to produce mechanical detail working in normal corrosive environments (organic acids, salt, alkali) in the food industry.
  • It is difficult for fabrication. We can improve by addition of selenium or increase content of sulfur but this will reduce corrosion resistance of steel.
  • This group will be corrosion in some specific cases: corrosion at the heat affected zone when welding or when often working in temperatures of 400 ~ 800 ° C. They will be focus corroded (point corrosion), corrosion under the stress and the cumulative effects due to corrosion and fatigue ... Because of the chromium carbide phase in the boundary reduce content of chromium and make them was corroded faster. We can be solved this matter by reducing the amount of Carbon to the lowest as possible level or adding stronger carbide elements than Cr as Ti, Nb, Ta, Mo ... Example: grade 316, 316L, 347 .

4. Stainless steel austenite-ferrite:

This type of steel is 18 ~ 28% Cr and 5 ~ 9% Ni, eg: Russia grade 12X21H5T.
Ferrite Austenite Dual Phases Stainless Steel Pipe
Ferrite Austenite Dual Phases Stainless Steel Pipe

The most important characteristics of this type of  stainless steel is very good mechanical properties and almost has no phenomenon of brittle ferrite of stainless steel, high elastic limit steel is over three times austenite. In addition, this group also has great corrosion resitance, especially in condition of high pressure (stress corrosion), or concentrated corrosion (point corrosion) and in the environment strong erosion (exhaust valve hole, pipe chemical vapor ...)

5. Stainless steel austenite-martensite (stainless steel hardening phase information):

This group has at least two advantages: can be processed by cold deformation and cutting in soft state. Beside, it can be hardened by aging in relatively low temperature to avoid deformation or oxygen processing. VD: 361 grade
Welding stainless steel by robot
Welding stainless steel by robot

Heat treatment regime:

  • Heated to 1050 ° C and then cooled in air. The product received is the austenite. This product can formed by plastic deformation and cutting in the cold state.
  • Heated to a temperature of 750 ~ 950 ° C and then cooled in air. The product received is austenite and carbide particles which quantity depends on heating temperature.
  • Cooled further to -75 ~ 0 ° C (cold) to transfer part or all of the austenite into martensite.
  • Artificial aging at ~ 525 ° C for 1h, the mechanical properties (strength, stiffness) will achieve the maximum value by extract very small elements Nial and Ni3Al (structural hardening)