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About Javad Mola
Expertise
I welcome questions related to physical metallurgy of steels preferably stainless steels.

Experience
As a PhD candidate at Graduate Institute of Ferrous Technology (GIFT), I am working on the formability of stainless steel sheets.

Organizations
Graduate Institute of Ferrous Technology affiliated to Pohang University of Science and Technology (POSTECH), Pohang, Republic of Korea. Pohang is home to POSCO, the third largest steel company in the world.

Publications
Solid State Phenomena, Materials Science and Technology

Education/Credentials
BS degree in Materials Engineering earned from Isfahan University of Technology, Isfahan, Iran. MS degree in Materials Science and Engineering earned from Sharif University of Technology, Tehran, Iran. Currently PhD candidate at Graduate Institute of Ferrous Technology (GIFT), Pohang University of Science and Technology (POSTECH), Pohang, Republic of Korea

You are here: Experts > Industry > Metals > Metallurgy > differences between 201, 202,304,316&430

Metallurgy - differences between 201, 202,304,316&430

Expert: Javad Mola - 10/17/2009

Question
i have similar problem that you answered in this address: http://en.allexperts.com/q/Metallurgy-2280/201-Stainless-Steel-8.htm.
i got good information from your answer, but i would be grateful if you give me more information about diferences of resistances of these kinds(oxidizing or corrosion). i want to buy some kitchen application( sink, gas hob, hood) and handrail( balustrade,..)that need cold work on them, in big quantity.

thanks

Answer

Composition of Selecte

I am very sorry for the delay in answering your question.

I receive a lot of questions related to the selection of the right stainless steel for the application of interest especially about the differences between 201 and 304 austenitic grades. Here I try to provide a more or less comprehensive answer which might be beyond what you are asking but it could nevertheless help those who got kind of similar problems and are looking for hints about material selection among the different available grades of stainless steels. For a more detailed understanding, one must refer to the available materials. The handbooks I know and recommend are the followings:

-   ASM specialty handbook on “Stainless Steels” edited by J.R. Davis
-   Handbook of Stainless Steels by D. Peckner and I.M. Bernstein (McGraw-Hill 1977)

If you cannot access these handbooks, the slides downloadable from the following address may still help. The pdf files can be of use if you already have a basic knowledge of stainless steels’ physical metallurgy.

Go to http://mdl.webteem.co.kr , click on “courses” on the left panel, then click on ”GIFT672 Stainless steel”. There are 13 pdf files: 1 Introduction+12 on different subjects related to stainless steels

To begin with, let’s have a look at the attached table which shows compositions of the stainless steels I may refer to throughout my explanations. Please keep in mind that the current steelmaking techniques allow to finely adjust the composition and thus different heats of a given grade may have considerable compositional variations, indeed within the relatively broad AISI range. Therefore, after making up your mind about the AISI grade that you are going to use, you still need to carefully select the heat which is more likely to impart the properties you expect to your product; even small differences in the chemical composition especially C and N contents can make two heats of a similar grade significantly different in terms of microstructure and properties.

The table only shows composition of the selected grades of austenitic, ferritic, and martensitic families of stainless steels. There are two more families namely “duplex”, and “precipitation hardenable” stainless steels which I will not discuss here in detail. I just confine to saying that duplex grades have a microstructure consisted of ferrite and austenite while precipitation hardenable grades might have a martensitic, austenitic, or semi-austenitic (austenite+martensite) matrix which can be further strengthened by formation of fine precipitates which harden the matrix.

- Factors controlling matrix in stainless steels:

Composition and heat treatment conditions control the matrix of stainless steels. Let me give you an example of how to make an austenitic grade. One must ensure that the following two conditions are met:

Condition 1- The composition and the annealing temperature (typically of the order of ~1000 °C) is so that a fully austenitic structure develops.

Condition 2- This austenitic structure remains austenitic after the part cools down to room temperature (RT).

To make sure condition 1 is met, a sufficient amount of austenite stabilizing elements must be contained in the alloy so that these elements overcome the ferrite stabilizing tendency of the ferrite stabilizers.

Main austenite stabilizers (Ni and its family): C, N, Ni, and Mn

Main ferrite stabilizers (Cr and its family): Cr (which is invariably above 10.5% in stainless steels), Si, Mo, Ti, and Nb

To satisfy condition 2, the martensite start temperature (Ms) must be below RT. This also depends on the steel’s chemistry. Ms can be roughly estimated by the available empirical formulae like the following (concentrations in mass percent):

Ms (in °C)=502-810C-1230N-13Mn-30Ni-12Cr-54Cu-6Mo         Eq. 1

Please note that the interstitial elements C, and N have a considerable effect on the Ms. This formula will only work if austenite composition is equal to the overall composition. In other words, if phases other than austenite are present at the annealing temperature, composition of austenite will no longer be equal to the overall cast composition. For instance, if some Cr23C6 exists at annealing temperature, the C and Cr contents of austenite would be less than the nominal composition of the alloy.

It is basically condition 2 which differentiates martensitic grades from austenitics; in martensitic grades condition 1 is met just like austenitics, but condition 2 is not, because Ms is above RT. Therefore, the austenite stabilized at the annealing temperature of martensitic grades transforms to martensite before RT is reached.

As previously mentioned, Ms equations similar to Eq. 1 will work only if composition of the austenite formed at annealing temperature is similar to the nominal composition of the alloy. As an example, if one uses the above equation to calculate the Ms temperature of the 440A type stainless (attached table), it will be below RT, so 440A is expected to be austenitic whereas it is martensitic in practice. The reason for this apparent discrepancy is coexistence of austensite and Cr23C6 carbides at the annealing temperature. Therefore, austenite is depleted of C and Cr which is associated with a rise in the Ms. So, Cr and C contents to be popped into Eq. 1 must be lower than the alloy’s nominal composition. In the case of austenitic stainless steels however, nominal and actual composition of austenite are more similar than high C martensitics, so simple insertion of nominal composition in Eq. 1 will lead to reasonably good estimations of Ms.

The austenite present in the austenitic stainless steels is metastable in the sense that it is not the phase predicted by thermodynamics. Therefore, it is possible to make it convert to martensite (partially or fully) by application of external force (in practice during cold forming operations). The martensite thus formed is called strain-induced martensite to differentiate it from the thermal martensite which forms right after quench from annealing temperature. Formation of strain-induced martensite will turn a deformed austenitic grade weakly or strongly magnetic (depending on the fraction of martensite which forms) since martensite is magnetic as ferrite is. By analogy with the Ms temperature which is used to estimate the temperature below which thermal martensite forms from austenite, an Md30 temperature could be defined for every austenitic grade which delineates the temperature at which application of 30% of true strain results in the transformation of 50% of the austenite to martensite (so Md30 is always above the Ms temperature). Therefore, the higher the Md30 temperature, the easier it is to form strain-induced martensite. There are simple equations like the followings (empirically derived) to estimate the Md30 temperature (concentrations in mass percent):

Md30 (in °C)= 413 -462 (C+N) -9.2Si -8.1Mn -13.7Cr -9.5Ni -18.5Mo          Eq. 2

Md30 (in °C)= 497 -462 (C+N) -9.2Si -8.1Mn -13.7Cr -20Ni -18.5Mo           Eq. 3

Therefore, solutions to prevent formation of strain-induced martensite are 1- decreasing the Md30 temperature by compositional modification, and 2- increasing the forming temperature from RT to slightly higher.

- Sensitization and its effects on the corrosion behavior:

Good corrosion resistance of stainless steels is due to the presence of high amount of Cr which leads to the formation of a protective corrosion resistant film on them. The higher the Cr content, the higher the corrosion resistance of the stainless steel is and any factor leading to local or uniform Cr depletion is deleterious to stainless steels. The critical Cr content needed to impart corrosion resistance depends on the environment in which they have to serve. If, due to processing conditions such as welding, stainless steels locally lose their Cr, so that Cr level falls to below the minimum required for corrosion resistance in that particular media, then such areas may act as potential corrosion initiation sites in a phenomenon called sensitization. Such a situation may arise for instance, when M23C6 precipitates form at grain boundaries due to exposure of the steel to the temperature range of 500-1000 °C. Local depletion of Cr from the areas surrounding precipitates (usually next to the grain boundaries) leads to sensitization of stainless steels. Susceptibility of stainless steels to intergranular corrosion is basically a consequence of sensitization.

Chromium carbide formation is not dangerous by itself. It is dangerous because it may lead to formation of a Cr-depleted zone right next to it. Therefore, if the temperature and exposure time to the chromium carbide forming range is so that after its formation, there is another chance for the Cr-depleted zone next to the carbide to recover its Cr content to above the critical level (by diffusion of Cr from farther regions to the carbides’ neighborhood), there is no need to worry about the sensitization. As a rule of thumb, sensitization is more deleterious if carbides are formed at lower temperatures and it happens easier in the ferritic grades than in the austenitics.

Low C content and addition of strong carbide formers like Nb, Ti, and Ta are effective ways of preventing sensitization (formation of carbides of elements other than Cr do not lead to sensitization).

- Comparison of selected stainless steel grades:

Austenitic stainless steels need a sufficient amount of austenite stabilizers to have an austenitic matrix at RT. Compared to the widely used 304, in 201 which is one of the oldest austenitic grades, the austenite stabilizer Ni is replaced by three other austenite stabilizers namely Mn, C, and N. Mn addition allows to incorporate higher N to the alloy because of increased solubility of N in presence of Mn. Although both 201 and 304 grades have an austenitic matrix, 201 is more susceptible to sensitization, because during cooling from high temperature processing operations like welding or heat treatment, C and N may locally tie up with Cr in the matrix to form precipitates like Cr2N, Cr23C6, and Cr7C3 (formation of Cr2N is so sluggish but the Cr23C6 carbide easily forms in high carbon stainless steels). Therefore it is generally safer not to use C as an austenite stabilizer. On the other hand, if the C level in 201 is low and it is not compensated by N, then presence of some ferrite at annealing temperature is inevitable, although sensitization will be less of a problem. The chances of strain induced martensite formation in response to cold deformation also increase with less of alloying elements especially C and N.

With 201, one must also be careful with the MnS formation when S level is high. If formed, such inclusions may act as corrosion initiation sites.

The 200 series of austenitic stainless steels have never found the wide acceptance anticipated at the time of their development in 1950s.

Nowadays, steelmaking industry is utilizing new techniques to further reduce the C levels of stainless steels in order to decrease their susceptibility to sensitization. As a result, low C versions of older grades have been introduced (304L is the low C version of 304). Addition of N to compensate for the reduced stability of austenite in 304L is also practiced in 304LN. N also imparts higher strength levels and when added with Mo, can have a synergistic effect on the pitting resistance. 316LN austenitic stainless for instance enjoys an increased corrosion resistance thanks to its low C, and the synergistic effect of Mo and N.

Compared to austenitics, ferritic stainless steels of the same Cr content show an inferior general corrosion resistance. They are on the other hand more resistant to stress corrosion cracking. At annealing temperature, ferritic grades (e.g. 409, 430) have a fully ferritic matrix, so they must have a low ratio of austenite to ferrite stabilizers for this to happen. Ferritic stainless steels cannot usually withstand humid environments and are thus used less extensively compared to austenitics.

Martensitic stainless steels are stronger than ferritics and austenitics but exhibit a lower general corrosion resistance. Their higher strength levels make them also more susceptible to hydrogen embrittlement. As mentioned elsewhere in the text, they are similar to ferritics in that they are both ferromagnetic and similar to austenitics because they are fully austenitic at annealing temperature. Their strength is specially controlled by their carbon content.

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