What is Stainless Steel?
This eBook provides engineers, purchasing agents, and plant personnel with a tool to enhance their knowledge of stainless steel and its uses as related to their present and future applications.
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Chapter 1
Chapter 1
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What is Stainless Steel? This is often the kind of statement heard from individuals when discussing a failure of process piping or equipment. This is also an indication of how little is actually understood about stainless steel and the applications where it is used. For years the food, beverage and pharmaceutical industries have used stainless steels in their process piping systems. Most of the time stainless steel components provide satisfactory results. Occasionally a catastrophic failure will occur.
The purpose of the information contained within this article is to bring an understanding to stainless steel, its uses, and why it will fail under certain conditions.
In the following paragraphs we will discuss:
- The different classes of stainless steel
- Heat treatment
- Corrosion
- Welding
- Material selection
As with any failure, it is imperative the cause of the failure be identified before a proper fix can be recognized. Most often the cause of the failure is identified as the wrong material being used in the wrong application. We can not solve problems by using the same kind of thinking we used when we created them.
This article provides engineers, purchasing agents, and plant personnel with a tool to enhance their knowledge of stainless steel and its uses as related to their present and future applications.
The Discovery of Stainless Steel
Stainless steel, the solid foundation of the sanitary industry, had a humble and practical beginning. In 1912 while searching for a solution to erosion on gun barrels caused by the action of heat and gases during discharge, stainless steel was first discovered at Brown Firth Laboratories.
However the first, true stainless steel was melted on August 13, 1913, and contained 0.24% carbon and 12.8% chromium. Building on the discovery from Brown Firth Laboratories, Harry Brearley was still trying to develop a more wear resistant steel. In order to examine the grain structure of this newly developed steel, he needed to etch the samples before examining them under the microscope. The etching re-agents Brearley used were based on nitric acid, and he found this new steel strongly resisted chemical attack. Encouraged, he then experimented with vinegar and other food acids such as lemon juice and found the same results.
Brearley quickly realized this new mix would have a greater product usage than just rifle barrels. He was from Sheffield, England, so he turned to a product the region was known for: cutlery. Up until this point, table cutlery was either silver or nickel plated while cutting knives were made from carbon steel that had to be washed and dried after each use. Then, the rust spots had to be rubbed off prior to the next use. On his own initiative, Brearley, found local cutler R.F. Mosley, introduced him to what he called “rustless steel” and had Mosley produce knives. When the knives would not stain in vinegar, it was Mosley’s cutlery manager Ernest Stuart who called the new knives “stainless steel.”
The following year in Germany, Krupp was also experimenting by adding nickel to a similar melt. The Krupp steel was more resistant to acids, softer and more ductile making it easier to work. From these two inventions in England and Germany just prior to World War I, the 300 series austenitic and 400 series martensitic steels were developed. However, the discoveries and credits came full circle in 1924 when Mr. Brearley’s successor at Brown Firth Laboratories, Dr. W.H. Hatfield invented 18/8 stainless steel. Containing 18% chromium and 8% nickel, it is commonly known today as 304 stainless steel.
What is Stainless Steel Made Of?
Stainless steel is not a single alloy, but a part of a large family of alloys with different properties for each member. The stainless steel family is quite large and specialized. There are hundreds of grades and sub grades, and each is designed for a special application.
What exactly is required for iron to be transformed into stainless steel? Chromium is the magic element.
Stainless steel must contain at least 10.5% chromium to provide adequate resistance to rusting. And, the more chromium the alloy contains, the better the corrosion resistance.
However, it is important to remember there is an upper limit to the amount of chromium the iron can hold. Because of this, additional alloying elements are necessary to develop corrosion resistance to specific medias.
By definition, stainless steel must contain a minimum of 50% iron. If it contains less iron, the alloy is named for the next major element. For example, if the iron is replaced with nickel, so the iron is less than 50%, it is identified as a nickel alloy
The magic element of chromium imparts a special property to the iron that makes it corrosion resistant. When the chromium is in excess of 10.5%, the corrosion barrier changes from an active film to a passive film. In this process, while the active film continues to grow over time in the corroding solution until the base metal is consumed, the passive film will form and stop growing. This passive layer is extremely thin, in the order of 10 to 100 atoms thick, and is composed mainly of chromium oxide.
The chromium oxide prevents further diffusion of oxygen into the base metal. However, chromium can also be stainless steel’s Achilles’ heel, and the chloride ion stainless steel’s nemesis. This is because in the passive layer, the chloride ion combines with chromium forming a soluble chromium chloride. As the chromium dissolves, free iron is exposed on the surface and reacts with the environment forming rust.
However, alloying elements like molybdenum will minimize this reaction. Other elements, as illustrated in the table to the right, may be added for specialized purposes.
For example: high temperature oxidation resistance, sulfuric acid resistance, greater ductility, high temperature creep resistance, abrasion resistance, or high strength. Again, of all these elements, only chromium is required for stainless steel to be stainless.
Classification
There are five classes of stainless steel: austenitic, ferritic, martensitic, duplex, and precipitation hardening. They are named according to how their microstructure resembles a similar microstructure in steel. The properties of these classes differ but are essentially the same within the same class.
AUSTENITIC
These are the most popular of the stainless steels because of their ductility, ease of working and good corrosion resistance.
All were derived from the 18Cr-8Ni stainless steels. Their corrosion resistance may be compared to the rungs on a ladder with Type 304 on the first rung and the other grades occupying the successive rungs.
What is 304 Stainless Steel?
The most common grade is Type 304 or 304L, which makes up over 60% of all the stainless steel made in the United States today. The other grades are developed from the 18–8 base by adding alloying elements to provide special corrosion resistant properties or better weldability.
For example, adding titanium to Type 304 makes Type 321, the workhorse of the intermediate temperature materials.
What is 316L Stainless Steel?
Adding 2% molybdenum to Type 304 makes Type 316, which has better chloride corrosion resistance. Adding more chromium gives
Type 310 the basis for high temperature applications. The major weakness of the austenitic stainless steels is their susceptibility to chloride stress corrosion cracking.
FERRITIC
Until the early 1980s, these alloys were not very popular because the inherent high carbon content made them extremely brittle and imparted relatively poor corrosion resistance. Research in the late 1960s, using vacuum electron beam melting, led to a new class of alloys sometimes called the “Superferritic Stainless Steels” of which E-Brite 26- 1‚® was the first. Then in the late 1970s a new steel refining technique, Argon Oxygen Decarburization (AOD), was developed.
This technique, together with the addition of titanium or niobium, allowed the commercial development of extremely corrosion resistant grades. Today SEA-CURE® stainless, one of the most popular superferritic alloys, is widely used in marine applications since its corrosion resistance in seawater is essentially the same as that of titanium.
The most widely used ferritic stainless steel is Type 409, a 10.5% Ce alloy with no nickel, used for automotive exhaust systems. Ferritic stainless steels are resistant to chloride stress corrosion cracking, and have high strength. Grades like SEA-CURE stainless have the highest modulus of elasticity of the common engineering alloys, which makes them highly resistant to vibration.
DUPLEX
Although these alloys were developed in 1927, their usefulness was not realized until the 1960s. They are characterized by having both austenite and ferrite in their microstructure, hence the name Duplex Stainless Steel. Duplex stainless steels exist in a narrow nickel range of about 4-7%.
A ferrite matrix with islands of austenite characterizes the lower nickel grades, and an austenite matrix with islands of ferrite characterizes the higher nickel range.
- When the matrix is ferrite, the alloys are resistant to chloride stress corrosion cracking.
- When the matrix is austenitic, the alloys are sensitive to chloride stress corrosion cracking.
High strength, good corrosion resistance and good ductility characterize them. One alloy, Carpenter 7-Mo PLUS‚® has the best corrosion resistance against nitric acid of any of the stainless steels because of its very high chromium content and duplex structure.
MARTENSITIC
These were the first stainless steels developed because of the inability to obtain low carbon steel. Basically, they are stainless tool steels because they use the same hardening and tempering mechanisms.
These grades are very common, from the blade in your pocket Swiss Army knife, to the scalpel the surgeon uses when he makes that first incision for a heart bypass operation.
Martensitic stainless steels are used in bearing races for corrosion proof bearings and other areas where erosion corrosion is a problem. These stainless steels are not especially corrosion resistant, barely as good as Type 304, but are infinitely better than the carbon steels they replace.
Like carbon tool steels, martensitic stainless steels derive their excellent hardness from the carbon added to the alloy. Their ability to maintain a keen edge comes from their high hardness and corrosion resistance.
PRECIPITATION HARDENING
These steels are the latest in the development of special stainless steels and represent the area where future development will most likely take place. They are somewhat soft and ductile in the solution-annealed state, but when subjected to a relatively low precipitation hardening temperature, 1000ºF (540ºC), their strength more than doubles and they become very hard.
The metallurgical structure of the common grades is martensitic, but some of the special high nickel grades are austenitic. The strengthening mechanism comes from the formation of submicroscopic precipitates, which are compounds of aluminum, copper, titanium, or molybdenum. These precipitates provide resistance to strain exerted on the structure.
The precipitates are so small they can be observed only at extremely high magnifications with special electron microscopes. Their action may be understood by the analogy of a deck of cards to a block of steel. When a force is placed upon the cards, the cards in the deck easily move in response to the force. If the block of steel is given the low temperature aging treatment, small precipitates form, similar to placing sea sand on the surface of the cards.
Now, it takes much more force to cause the cards to move; so, the material is much stronger. The primary use of precipitation hardening steels is where high strength and corrosion resistance are required. Aerospace and military applications have dominated the applications in the past, but new uses in instrumentation and fluid control are being found.
NICKEL BASED ALLOYS
Many consider these alloys to be stainless steel. But if you recall, by definition in order for stainless steel to be stainless steel it must contain a minimum of 50% iron. The iron ratio in nickel based alloys are considerably less than 50%. Within nickel based alloys there are four classifications.
- Group A is nickel and nickel-copper alloys such as Monel 400
- Group B is Chromium bearing alloys as in Hastelloy® C-22® and C-276
- Group C is Nickel Molybdenum alloys such as Hastelloy B2, B3 and B4
- Group D is Precipitation-hardening alloys as are Monel K-500 and Inconel alloy 718.
The wide range of nickel based alloys available are used for their resistance to corrosion and retention of strength at elevated temperatures. Many severe corrosion problems can be solved through the use of these alloys. However, they are not universally corrosion resistant. The proper alloy must be carefully selected for a specific application.
Nickel and nickel based alloys are very resistant to corrosion in alkaline environments, neutral chemicals and many natural environments.
In addition, many nickel based alloys show excellent resistance to pitting, crevice corrosion and stress corrosion cracking in chloride environments.
STRENGTH & HEAT TREATMENT
The alloy strength is controlled by the chemical composition and the metallurgical structure. Only the martensitic and precipitation hardening stainless steels can be heat treated to obtain higher strength.
- Strengthening, or an increase in the ultimate and yield strengths, of the other grades must be achieved by cold working the structure.
- Heat treatment of the austenitic, martensitic and duplex grades is used to remove residual stress and, in the case of the austenitic stainless steels, to reduce the probability of chloride stress corrosion cracking.
Heat treatment is also used to dissolve any undesirable metallurgical phases that may be present. Heating and cooling the various grades of stainless steel must be done with caution. Be very careful using acetylene, MAP or propane torches to heat the stainless steel. If a reducing flame is used, excessive carbon may be transferred to the metal resulting in the formation of chromium carbide and ultimately, failure of the part.
Before attempting heat treatment of a particular grade of stainless steel, always refer to the heat treatment data for that particular grade.
For example, slow cooling a high carbon austenitic stainless steel from the solution anneal temperature may lead to precipitation of chromium carbide. This will result in poor corrosion resistance and low ductility.
Holding a ferritic or duplex stainless steel within the 885ºF (475ºC) embrittlement temperature range which can be as low as 600ºF (315ºC)-may lead to brittleness at room temperature. Heating high chromium, high molybdenum austenitic stainless steel to a temperature below the specified minimum heat-treating temperature, may lead to precipitation of second phase compounds along the grain boundaries. When placed in service, these alloys may corrode or fail because of low ductility problems. Always check on the nature of the alloy before attempting any type of heat treatment.
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This eBook provides engineers, purchasing agents, and plant personnel with a tool to enhance their knowledge of stainless steel and its uses as related to their present and future applications.