February 24, 2017
What are 'L' grades stainless steels ?
Nickel alloys, markets and uses.
Can I use stainless steel at high temperatures?
What is stainless steel?
What are 'L' grades stainless steels?
There are a number of variations of Type 304 and Type 316 stainless steel. The basic difference is the carbon content allowed in these variations. The following table shows the differences between 304, 304L and 304H.
||0.0 - 0.08
||0.0 - 0.03
||0.04 - 0.10
|0.0 - 0.03
Carbon content has an effect on room temperature strength as seen by the lower strength levels in 304L. Higher carbon contents improve high temperature strength, which is purpose of the minimum carbon content of 0.04% in 304H.
Unfortunately, carbon content in excess of 0.03% can reduce corrosion resistance in the heat affected zone of welds. When a stainless steel such as Type 304 or Type 316 with carbon content in excess of 0.03% is heated into the temperature range of 500 to 850 deg C there is the possibility for the precipitation of chromium carbides along the grain boundaries of the steel. This phenomenon is called sensitization. Sensitization can deplete the chromium available along the grain boundaries reducing corrosion resistance, which can result in intergranular corrosion. Fortunately, the ease at which a stainless steel can be sensitized is dependent on carbon content and time. With a carbon content of 0.05% a stainless steel can be sufficiently sensitized and suffer intergranular corrosion in less than 10 minutes of exposure to the aforementioned temperature range. However, at 0.03% carbon it would take almost an hour and at 0.02% carbon it would take almost 10 hours. Thus Type 304 and Type 316 stainless steels intended for corrosion service are specified with a carbon content maximum of 0.03% and are designated as 304L and 316L to indicate their low carbon content.
Unfortunately, as we can see in the table above "L" grade possesses lower strength. Fortunately, with modern steelmaking practice it is possible to produce dual certified stainless steel which meets the higher strength of 304 but possesses the lower carbon content of 304L by simply adding a small amount of nitrogen, which is allowed by the specification. Thus it is possible to produce dual certified 304/304L. The same is possible with 316/316L.
Nickel alloys, markets and uses
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Additional information about specific nickel bearing alloys and their uses can be found on the websites of several nickel alloy producers, such as
Special Metals - http://www.specialmetals.com
Haynes International - http://www.haynesintl.com
VDM Metals - http://www.vdm-metals.com
Allegheny Technologies - https://www.atimetals.com
Can I use stainless steel at high temperatures?
There are many factors that can affect the use of a metallic material at high temperature. The most common requirement is resistance to scaling in high temperature air, which results in the conversion of the metal into oxide. Comparing stainless steel to carbon steel, which is limited to about a maximum temperature of about 500oC (950oF), any grade of stainless will easy resist scaling at this temperature. Thus in this case we can say stainless steels are well suited to withstand high temperatures. However, there are many different grades of stainless steel and thus there is varying oxidation resistance. Increasing amounts of chromium impart greater oxidation resistance. The minimum chromium content allowed in a stainless steel by definition is 10.5%, while some grades may have as much as 28%. A stainless steel with 10.5% chromium would be resistant to about 700oC (1300oF), while a grade with about 25% chromium would be resistant to about 1150oC (2100oF).
Stainless steels can be grouped into five different families primarily based on their microstructure. These families are:
- Precipitation Hardenable
Some families are better suited for use at high temperature than are others. This suitability is determined primarily by its microstructure.
Austenitic stainless steels, which are the largest family by tonnage, possess an austenitic microstructure, which is due to a significant addition of nickel. Nickel additions can range from 8% in Type 304 stainless steel to 20% in Type 310 stainless steel. Type 304 is the most common grade of stainless steel, which is found in household items such as pots, cutlery and kitchen sinks. Type 304 possesses about 18% chromium and is considered oxidation resistant up to about 870oC (1600oF). Type 310 with a minimum of 25% chromium is commonly used in high temperature industrial applications up to 1150oC (2100oF). Increasing chromium contents require increasing nickel contents to maintain an austenitic microstructure. All of the grades in the other families have no or very little nickel addition.
All of the families except Austenitic are highly susceptible to formation of detrimental microstructural phases when exposed to the temperature range of 480oC (900oF) to 870oC (1600oF), which will embrittle the material when it returns to room temperature. Their toughness can be so compromised that they are easily fractured by slight impact. Austenitic stainless steels are, in comparison, highly resistant to the formation of these detrimental phases.
Also, the austenite phase provides the greatest resistance to creep, which is a time-dependent deformation at elevated temperature and constant stress. This means that metals behave differently at high temperatures than they do near room temperature. Thus if a metal bar is loaded to just below its yield strength at room temperature, that load can be left there practically forever. Nothing will happen, unless it corrodes away or stress-corrosion cracks. Now let us say that this metal bar is loaded, again keeping the stress below the yield strength, but at a temperature above its creep temperature. The metal bar will begin to stretch, but very, very slowly (it creeps). It will keep on stretching for hours, weeks, maybe years, until it finally breaks in two. All this, when it wasn’t even loaded up to the yield strength. The temperature at which creep begins depends on the alloy composition, but for a stainless steel it starts at about 565oC (1050oF). As temperature increases the speed at which the metal creeps also increases.
Creep is also strongly influenced by the material’s carbon content. Carbon contents in excess of 0.04% provide superior resistance to creep and thus even within the austenitic family the various grades of stainless steel can be further subdivided based on their carbon content. Carbon contents in excess of 0.03% can impair the corrosion resistance of stainless steels in the weld area. Thus austenitic stainless steels with carbon contents less than 0.03% are typically used for corrosive applications, but they would still possess greater resistance to creep than any stainless steel in the other four families. While austenitic stainless steels with carbon content in excess of 0.4% possess the highest creep resistance.
Ferritic stainless steels are the second largest family by tonnage. The low chromium grades containing only about 11% chromium, commonly known as Type 409, are much less susceptible to formation of detrimental microstructural phases than the higher chromium grades of this family. Type 409 is the most common grade in this family and is used for example in the manufacture of automotive exhausts.
Duplex stainless steels represent only a very small tonnage of stainless steel production. Their microstructure is balanced between approximately equal amounts of ferrite and austenite phase. Their development has been directed exclusively for highly corrosive applications, though their high chromium contents would provide excellent oxidation resistance. However, their ferrite phase makes them highly susceptible to formation of detrimental microstructural phases and makes them a poor choice for high temperature applications.
Finally, Precipitation hardenable and Martensitic stainless steels are strengthened by heat treatment, but this strengthening will be compromised when exposed to high temperature.
Thus all stainless steels possess excellent resistance to scaling above 500oC (950oF) where a carbon steel is unsuitable. However, austenitic stainless steels containing nickel are the most common metallic material used at high temperature and are also used at lower temperatures, even those found in kitchen stoves.
There are also nickel alloys, with nickel contents in excess of 40%, which also possess an austenitic microstructure that have been developed exclusively for high temperature applications which possess even greater resistance to deformation at elevated temperature and are immune to detrimental microstructural phases.
What is stainless steel?
Stainless steel is an alloy of Iron with a minimum of 10.5% Chromium. Chromium produces a thin layer of oxide on the surface of the steel known as the 'passive layer'. This prevents any further corrosion of the surface. Increasing the amount of Chromium gives an increased resistance to corrosion.
Stainless steel also contains varying amounts of Carbon, Silicon and Manganese. Other elements such as Nickel and Molybdenum may be added to impart other useful properties such as enhanced formability and increased corrosion resistance.