Technical information

Stainless Steel Types

There are different types of stainless steel. For example, when nickel is added, the austenitic microstructure of iron becomes stable. This crystal structure steel makes it a non-magnetic, less brittle steel at low temperatures. The amount of carbon it contains is increased for higher hardness and strength. Along with heat treatments, these steels can be used in many products such as razors, knives and inserts. Mangan is also found in many steels in different proportions and helps maintain the austenitic structure given by nickel at lower costs.

            Stainless steels are classified in five groups according to their crystal microstructure.

1. Austenitic Stainless Steels

2. Ferritic Stainless Steels

3. Duplex Stainless Steels

4. Martensitic Stainless Steels

5. Precipitation Hardened Stainless Steels (PH)

1. Austenitic Stainless Steels

2. Ferritic Stainless Steels

Ferritic stainless steels are generally a stainless steel group that does not contain nickel, contains high chromium (between 10.5% and 30%), carbide-forming materials such as molybdenum, titanium vanadium and alloy elements that stabilize the ferritic structure.

The high chromium content they contain in general provides ferritics with a very high corrosion resistance. Ferritic stainless steels, which have mechanical and physical properties close to the properties of carbon steels, which are mostly close relatives, are magnetic in contrast to austenitics, they cannot be heat treated due to their low carbon content and can be easily rolled. The only heat treatment that can be applied to such steels is annealing.

Recently, the extreme price rise and change in alloying elements, especially nickel, has accelerated the development of ferritics, and new, wide-ranging and cost-effective ferritic grades have been developed, as well as corrosion-resistant as well as austenitics at low cost.

3. Duplex Stainless Steels

The performance of these steels, which contain ferrite and austenite in their micro structures, in equal proportions, varies according to the alloys they contain. Duplex stainless steels have a higher strength than austenitic stainless steels, but they have a better resistance against austenitic corrosion, especially against nucleation, crack and stress corrosion. Duplex grades are also more durable thanks to the high rate of chromium, which is between 19% and 28%, molybdenum up to 5%, and nickel content, which is lower than that of austenitics. The most important restrictive feature of Duplex stainless steels is their fragility at high temperatures and very low temperatures. Especially if working over 300 ° C and below -50 ° C for a short period, duplex steels become brittle and need to anneal again. The most commonly known duplex stainless steel grade is 2205 grade. Their structures are BCC for their ferritic parts and FCC for their austenitic parts.

4. Martensitic Stainless Steels

5. Precipitation Hardened Stainless Steels (PH)

Precipitation-hardened stainless steels, also called "aging-hardened stainless steels," are basically a type of stainless steel that contains chromium and nickel, combining the properties of both in an appropriate way between martensitic and austenitic grades.

Just like martensitic stainless steels, they can gain high strength with heat treatment and they have corrosion resistance like austenitic grades. Hardening is achieved by adding one or more of the alloying elements such as copper, aluminum, titanium, niobium and molybdenum. The most commonly known quality in this group is 17-4 PH. This quality is also known as 630. This quality, named after 17% chromium and 4% nickel, also includes 4% copper and 0.3% niobium.

An advantage of precipitate hardened stainless steels is that these materials can also be supplied under "treated solution" conditions, ready for mechanical work, processing. After mechanical processing or production, the strength of the steel can be increased as desired by very simply applying a low temperature heat treatment. Since this process is done at low temperature, the material produced or applied does not cause temperature distortions or distortions.

Precipitation hardened stainless steels are divided into three subgroups: Martensitic PH, semi-austenitic PH and austenitic PH.

PH stainless steels can be corrosion resistant up to austenitic 304 quality, depending on the alloy ratio. Corrosion resistance is very low in annealed condition. Therefore, it should not be used before heat treatment. Their structures can also be BCT or FCC or both, depending on their subgroups.

304 Quality Stainless Steel

ASTM 304 (304 Quality) is the biggest stainless steel success story. It accounts for 50% of all stainless steel production and about half of stainless consumption and is used in almost all industrial applications.

304 is not just a stainless steel that can suit every application; it also provides an excellent basis for understanding the qualities of 304, a practical basis for defining the suitability of stainless steel in a desired application, and for comparing the materials of the austenitic stainless steel group. We all have a satisfactory experience with the use of 304 and knowledge of its deep drawing properties. The metallic part used in your cutlery sets (see markings 18/10 and 18/8), pressure cookers, sinks and even floppy disks is made of 304 stainless steel.

Components:

Grade 304L (Please see Table-1) is a low carbon 304 that is used occasionally to prevent possible corrosion sensitivity in welded parts. Quality 304H (Please see Table-1), increases the strength (especially at temperatures above 500 ºC) and contains a higher carbon than 304L. This quality is not used for applications with the possibility of sensitive corrosion.

Note-1: The% rates not given as a range represent the maximum rates.

Note-2: These values ​​are defined in ASTM A240 for plate, sheet and roll. For some other products, the identification may differ slightly from these values.

Both 304L and 304H are suitable for plate (flat product) and pipe, but 304H may be less available before stock. 304L and 304H are sometimes stocked like standard 304. (Test certificates confirm whether this is "L" or "H")

Corrosion Resistance:

Quality 304 has excellent corrosion resistance in a wide area. It does not rust in many architectural construction applications. In addition, it is easily cleaned and resistant to organic chemicals, inorganic chemicals and colored dyes in a wide area in many food production and processing environments.

Grade 304 is subjected to stress corrosion cracking and nucleation and cracking corrosion in medium temperature chlorite environments where tensile strength is applied at temperatures above 50 ºC. In addition, it can be successfully applied at short intervals and in warm chlorite environments where cleaning is regularly performed and applied (eg in pots and some yacht connections).

Heat Resistance:

It has a good oxidation resistance at places where 304 Quality is worked at short intervals at 870 ºC and continuously worked at 925 ºC. It is not recommended in the range of 425 - 860 ºC of 304, if the subsequent application is working in aqueous environments at room temperature. But sometimes it performs well in environments that change above or below this temperature range.

Grade 304L is more resistant to carbide precipitation and can be used in the temperature range mentioned above. Where high temperature material strength is important, high carbon values ​​are needed. For example, AS1210 pressure vessels code limits the operating temperature of 304L to 425 ºC, and the use of 304 quality is restricted to values ​​of 0.04% and higher for temperatures above 550 ºC.

Quality 304 has an excellent toughness at low temperatures of liquefied gases and application at these temperatures is also available.

Mechanical and Physical Properties:

Like other austenitic grades, it is almost non-magnetic at annealed form 304. (very low magnetic property). But after cold rolling, it can have a significant magnetic property. (Can be reversed with annealing)

As with other austenitic steels, it can only be hardened in 304 by cold rolling. Tensile strength values ​​exceeding 1,000 MPa can be achieved and depending on the type of product requested, a special cold rolled, high-strength order can also be ordered. (See ASTM A666 or EN 10088-2)

Annealing process is the main heat treatment applied in 304 quality. This is done by heating up to 1,010 - 1,120 ºC and cooling quickly - generally immersed in water.

manufacturability

Quality 304 has a perfect formability. It can be used in deep drawing works without any intermediate softening operation with heat treatment. This feature causes this material to be preferred in the production of products such as pots and pans used in the production of deep drawing (plastering in the local language). It can be easily cut, shaped and used for other types of parts in the fields of industrial, architectural and transportation.

304 quality is also suitable in terms of weldability and all standard welding techniques can be used. (Although Oxyacetylene is not normally used) Although proper cleaning after welding is recommended, post-welding annealing is sometimes not necessary to maintain the corrosion resistance of 304. It does not require annealing process after welding 304L and it finds a large place in heavy dimension production.

The machinability of 304 is lower than that of many carbon steels. Standard austenitic steels like 304 can be easily machined at slow speeds and with heavy feed, using hard and sharp tips and coolant.

Price Comparison:

Although only "Initial Cost" price comparison is not suitable, it is recommended to use sheet materials on standard rolling surfaces for the guideline, construction projects in Table-4. The "Cost of Life" parameters significantly increase the attractiveness of stainless in many applications compared to other "initial cost" competitors.

Suitable Forms:

It is possible to obtain 304 grades in almost all stainless product forms, such as roll, sheet, plate, strip, tube, pipe, fitting, bar, angled product, wire, casting and other shapes. Also, it is possible to find 304 in all surface qualities produced from stainless steel, from standard to special surfaces.

Apps:

In certain cases and applications such as sea conditions, temperature conditions exceeding 50 - 60 ºC, environments with chlorite, situations requiring heavy welding, situations requiring multi-processing, high strength, hardness or strips obtained by cold rolling, to 304 alternative qualities should be evaluated.

However, kitchenware, architecture, food and beverage process, tool-equipment, commercial and home kitchen production, boilers, chemistry, petro-chemistry, mineral processes and other industries are the typical applications of 304.

With this wide range of uses, 304 quality has become a basic material in the modern industry.

316 Quality Stainless Steel

If an application requires a higher strength than the corrosion resistance 304 quality will provide, 316 grade is a step ahead. Quality 316 has a mechanical, physical and manufacturability character visually equivalent to 304 quality, but has a better corrosion resistance than 304 quality, especially against nucleation corrosion in chlorite environments.

It is the second most popular grade in the 316 grade stainless steel family. It has a 20% consumption rate among all manufactured stainless steel products.

Components:

Table-1 shows the comparison of 316, 316L and 316H grades.

It is a 316L low carbon 316 and is used against sensitive corrosion in welded parts.

316H quality contains a higher rate of carbon than 316L and has high strength (especially at temperatures above 500 ºC). However, it should not be used in places where sensitive rust can be seen.

Corrosion Resistance:

Grade 316 has a wide range of uses and excellent corrosion resistance. Its main advantage compared to 304 quality is its increased resistance against nucleation and cracking corrosion in warm chlorite environments. It is resistant to rusting that may occur in all architectural applications and is sometimes used under very difficult conditions such as sea-view structures, bridge connections and beams. It is very preferred in food processing environments due to its easy cleaning. It shows resistance to organic chemicals, dyes and a wide range of inorganic chemicals.

Stress corrosion breaks occur in hot chlorite environments when exposed to 316 quality nucleation and cracking corrosion and tensile stress at temperatures above 50 ºC.

In such difficult conditions, duplex grades such as 2205 (UNS S 31803) or high-alloy austenitic grades (UNS S31254) containing 6% molybdenum are the appropriate choice. Corrosion resistance of 316 quality high or low carbon sub-versions (316L and 316H) is similar to standard 316. These sub-versions are generally preferred because of their better resistance in welding (316L) or for high strength (316H) at high temperature.

Heat Resistance:

Like 304 quality, 316 quality has good oxidation resistance under intermittent application conditions at 870 ºC and continuous application conditions at 925 ºC. Exposure of 316 quality to a continuous working environment between 425 ileC and 860 ºC is not recommended if it continues as an aqueous environment at room temperature after application. However, these grades can sometimes perform well at temperatures that vary above or below this specified range.

The 316L grade is more resistant to carbide precipitation than the standard 316 grade and 316H and can be used in the high temperature range. However, high carbon values ​​are needed where high temperature strength is important. For example, the pressure vessels standard limits the operating temperature of 316L to 450 450C and does not allow 316 quality carbon value to be 0.04% or higher for temperatures of 550 sıcaklıkC. The version containing 316H or titanium can be adapted for 316Ti high temperature applications.

Like other austenitic stainless steels, 316 have excellent toughness at low temperatures of liquefied gases, and although lower cost grades such as 304 are generally preferred for cryogenic containers, they have applications at these temperatures.

Mechanical and Physical Properties:

Please see Table-2 and Table-3.

Table-2: Mechanical Properties of 316 Grade given in ASTM A240M (annealed) Note: In other standards, features are given slightly differently.

Table-3: Physical Properties of 316 Quality (Typical properties of annealed material) Like other austenitic grades, 316 is almost non-magnetic. 304 grade (such as very low magnetic permeability) is significantly magnetized by cold rolling, while 316 grade is hardly affected. This feature may be a reason to prefer it in some applications. The common feature of 316 quality with other austenitic grades is that it can be hardened only by cold rolling. Breaking strength exceeding 1.000 MPa can be achieved depending on the shape of the requested product and quantity. It can also be ordered according to the cold rolling strength provided that it is specially defined. (See ASTM A666 or EN 10088-2) Annealing (also called solution process) is the main heat treatment applied in 316 grades. This is achieved by rapid cooling (usually by immersion in water) by heating to 1010 -1120 oC.
 

manufacturability:

Like other austenitic grades, it has a perfect formability at 316. As in the production of deep drawing stainless parts such as washbasin, pots, deep drawing can be done without subjecting intermediate heat softening. However, no extra corrosion resistance of 316 grade is required for normal applications. 316 quality can be used in industry and architectural applications by easily bending and rolling many types of parts. 316 quality has a suitable welding feature and all standard welding techniques can be applied. (Although oxyacetylene is not normally applied) Post weld annealing makes it more suitable for heavy-duty productions, although it is sometimes not necessary to maintain the corrosion resistance of 316; a suitable post-weld cleaning is recommended. The machinability of 316 is lower in many carbon steels. Like standard austenitic grades, 316 can be easily machined with hard and sharp tips, if slow feeding, at low speeds and using coolant. There are also versions with improved machinability of 316.

Price Comparison:

The values ​​in Table-4 are the approximate initial cost comparison of the flat product of standard surface quality for building projects. Considering the cost of life, the attractiveness of stainless increases incredibly compared to its initial cost.

Table-4: Initial-Cost Comparison

Suitable Forms:

It is possible to find 316 grades in almost all stainless product forms, such as roll, sheet, plate, strip, tube, pipe, fitting, rod, bar, angled product, wire, casting and other forms. In addition, it is possible to find 316 in all surface qualities produced from stainless steel, from standard to special surfaces.

Apps:

We can summarize the typical application of 316 as follows: in yacht connection and building elements, especially in architectural structures at sea, dirty or industrial environments, equipment for food and beverage processes, hot water systems and chemical, petrochemical, mineral process, photographic and other industry applications.

Sometimes 316 quality is defined as marine quality and it is an upper step of 304 base quality.

Alternatives:

In some of the following applications and environments, alternative quality options to 316 quality should be considered:

In strong reducing acids (904L, 2205 or super duplex grades may be an alternative)

In environments above 50-60 ºC and containing chlorite (choose grades with high stress corrosion breakage and high nucleation corrosion resistance such as 2205 or super duplex or super austenitic)

Applications requiring heavy welding (316L), additional machining (improved machinability of 316), high strength or hardness (martensitic or precipitation hardened grades)

Austenitic Stainless Steels with Low Nickel

304 and 316 grades are the most recognized grades among stainless steels, especially because of their combination of properties such as excellent corrosion resistance, mechanical and physical properties and ease of production caused by austenitic microstructure. The austenitic structure is formed by adding a nickel of approximately 8-10%. Nickel is not the only element that forms the austenitic structure. Other elements used for this purpose are manganese, nitrogen, carbon and copper. The Cost of Nickel and Its Addition to Stainless Steel: In general, the cost of stainless steel is determined by the cost of the component alloys. Chromium, which is the basic component of stainless steel and against the corrosion, creates a chrome oxide layer on the surface and prevents corrosion, the cost of chromium is not high, but adding elements such as corrosion resistance (especially molybdenum) or ease of production (especially nickel) increases the cost very much. The cost of Nickel was at the level of $ 5,000 - $ 6,000 / ton in 2001. At the end of May 2007, this figure increased to $ 50,000 / ton. Similarly, the price of molybdenum, which was around $ 8,000 / ton in 2001, has now risen to $ 40,000 / ton.

These cost increases had a direct impact on these two grades: 304 (18% Cr, 8% Ni) and 316 (18% Cr, 10% Ni, 2% Mo). Of course, the quality most affected was 316. In addition, other stainless steels with high alloying elements such as 2205 (22% Cr, 5% Ni, 3% Mo) duplex grade are also affected. The role of the alloying elements is mainly to change the microstructure which will affect certain changes or mechanical and manufacturing properties for corrosion resistance. A general approach used to determine the corrosion resistance is the Nucleation (Pitting) Resistance Equivalent Coefficient (PRE). PRE =% Cr + 3,3xMo + 16x% N. PRE coefficient is used to show the resistance of grades against nucleation corrosion and put them in a ranking for this purpose. It cannot be used to reveal the condition affecting any corrosion. As can be seen, besides chromium, molybdenum and nitrogen have an effective resistance to this type of corrosion.

Although nitrogen can be costed much cheaper than molybdenum and nickel, its effect on corrosion resistance is limited as its solubility in steel is limited to 0.2%. The microstructure of the steel depends on the balance between the ferrite and austenite forming elements. As austenite building elements, carbon, manganese, nitrogen and copper elements are alternative to nickel. All these elements cost less than nickel. As seen in the PRE formulation, each element acts in different ways and it is not possible to completely remove nickel in austenitic grades.

Although not as effective as nickel, Mangan is a stable swelling element of austenitic structure and Cr-Mn steels have a higher rolling hardening feature than Cr-Ni steels. Although not specified in the PRE formula, nickel has a positive effect in conditions that cause much more corrosion than the action of manga. There is also a synergy between the elements. While nitrogen is very effective in stabilizing the austenitic structure, while manganese itself stabilizes the austenitic structure, it also has an effect to increase nitrogen dissolution in steel.

Rise of 200 Series Stainless Steels:

Manganese is an important alternative to nickel, from minor additions to substantial replacement. The development of high manganese austenitic steels occurred approximately 60 years ago, when nickel prices increased excessively. At these times, Cr-Mn-Ni grades such as 201 (17% Cr, 4% Ni, 6% Mn) and 202 (18% Cr, 4% Ni, 8% Mn) are alternatives to 301 and 302, which are chrome-nickel grades. They are the grades that have taken place in AISI and are still being produced and used. Their consumption is lower compared to Cr-Ni equivalents recently. The reasons for the low demand of these qualities can be listed as follows:

High rolling hardening rate (this may be an advantage in some applications)

Much lower surface quality properties are not suitable for some applications.

Additional production costs, high refractory wear during smelting

Corrosion resistance is lower in some working environments compared to Cr-Ni grades.

In another subject, Cr-Ni and Cr-Mn-Ni austenitic grades are not magnetic, while the scrap dealers determine the scrap value based on the approximate nickel value it contains.

Recent developments:

We see that Cr-Mn-Ni austenitic grades have improved recently. The most important development is in India, and its use in utensils and cooking equipment is increasing. The suitability of low nickel high manganese qualities to high rolling hardening is acceptable in these applications, and the addition of copper is used to reduce this problem. Due to local economic factors, India achieves appropriate results in the development and production of these applications. Likewise, Asian countries have a strong market in terms of these qualities and have recently increased their production. Production and usage of these qualities, which find a strong demand especially in the Chinese market, are increasing day by day. These grades are produced in countries such as Taiwan, Brazil, and Japan, and their nickel content can vary from 1% to 4% and manganese contents up to 9%. None of these have yet been included in ASTM or other international standards. The increase in the production of low nickel austenitic grades is very fast. According to ISSF data in 2003, the production of these grades constitutes 7.5% of the total world stainless steel production with 1.5 million tons. It is estimated that these qualities are 25% of the 2004 production rate in China. These figures are thought to constitute a much higher production rate today, especially due to the nickel prices that have skyrocketed since 2006.

Carbon Content

General austenitic stainless steels such as 304 and 316 are also produced with controlled low and high carbon content known as "L" and "H". Low carbon or "L" grades are used to reduce the sensitivity level of stainless steel at high temperatures and to reduce or prevent corrosion. The problematic temperature range encountered in welding or special applications is between 450 and 850 ºC. "L" grades are generally available in the form of flat materials over 5 mm thick.

High carbon, "H" grades are used in applications that require higher strength. The use of "L" and "H" grades interchangeably is a common occurrence.

What Are "L" Grades And Where Are They Used?

"L" grades are used where there is high temperature application, including the source of medium and large materials. Low carbon is one of the ways to prevent or retard intergranular carbide precipitation (often referred to as "sensitivity") that can cause intergranular corrosion in corrosive environments. The precipitation of carbides in the temperature range from 450 to 850 ºC has an incubation period. Since the time required for precipitation to occur largely depends on the carbon content, the low carbon content increases the resistance to this problem. Due to their application, "L" grades are available as plates, sheet metal, pipes and often round bars. Where there is no high temperature application or heavy welding requirement, "L" grades are generally identical to others.

What Are "H" Grades And Where Are They Used?

"H" Grades are high-carbon versions of standard grades and have increased strength, especially at high temperatures (usually above 500 ºC). The "creep resistance" values ​​formed with long-term applied loads are high. "H" grades are mainly produced as plates and pipes. In general, the grades to which it is applied are found in high-carbon versions of 309, 310, 321, 347 and 348, as defined in ASTM A240 / A240M, as well as 304H and 316H. These grades are greatly affected by carbide precipitation, which is called "sensitivity" if they are kept in the temperature range of 450 - 850 ºC. If the "sensitivity" problem occurs, the ductility and toughness at normal temperature will drop and their corrosion resistance will be significantly lost.

What is the difference?

The components of 304 and 304L grades are equivalent except for the carbon content. In theory, the maximum nickel content of 304L quality is allowed up to 12%, while this ratio is maximum 10.5% in standard 304 quality. However, due to the high course of nickel prices, nickel is generally used at a minimum of 8.5%, which is the lower limit in these two grades. The carbon lower limit has not been defined in either quality. A material with 0.02% carbon content complies with the requirements of both 304 quality and 304L quality.

Difference of the standard 304 quality from the chemical components except that the 304H quality carbon content is limited to 0.04 - 0.10% (note the minimum definition of the carbon content) and the 304H quality does not have a maximum limit of 0.1% as in the nitrogen content standard and 0.1%. They do not. In addition, all austenitic "H" grades should have a minimum grain size of 7 or greater as ASTM grain width.

The relationship between 316, 316L and 316H grades is the same as in 304 quality. Among these grades, there are only restrictions on carbon content, nitrogen content and grain size. In Table-1, you can see the carbon contents of the grades taken from ASTM A240 / A240M. In some other product specifications, especially pipe and tube specifications, the carbon upper limit limitation for 304L and 316L is maximum 0.035% or 0.040%. Other specifications of the specifications are the same.

In Table - 2 you can find the specifications of the mechanical properties taken from ASTM A240 / A240M. Practically, it pays particular attention to producing steel rollers to meet the standard quality requirements of "L" quality productions. For example, the breaking and yield strength values ​​of all 304L grades are above 205 and 515 MPa. Thus, they produce materials that address a wide market that meets both standard and "L" quality requirements.

5. The dimensions and other requirements are the same for standard, "L" and "H" grades.

6. Specifications such as pressure vessel specification and pipe pressure specification impose a workable pressure range limit for each quality at high temperatures. These specifications do not allow the use of "L" grades at high temperatures above 425 ºC. In addition, these specifications impose a restriction on the use of temperatures above 550 şeklindeC to contain at least 0.040% carbon. Therefore, these specifications do not allow the use of 304 and 316 grades containing 0.020% carbon in these applications, regardless of whether they are "L" or not. As long as the standard and "L" grades meet the chemical components and mechanical properties specified in the specifications, they can be used in the range from room temperature to the limited temperature of "L" grades.

Pressure vessel specification allows to be used with standard grades as long as it complies with "H" grades and desired strength values.

Alternative Quality Usage:

Where conditions require, standard, "L" and "H" grades can be used interchangeably.

This situation depends on the following conditions:

"L" grades can replace standard grades in case of mechanical properties and in applications that do not require high temperature resistance. "L" grades generally meet the requirements of standard grades. However, the manufacturer's certificate should be carefully checked for each feature and its suitability confirmed. It is very common for manufacturers to produce or deliver "L" quality versus standard quality orders. In practice, there will be no problems, as long as there is a use in accordance with its specifications and the end-user and the part manufacturer have no problems.

Standard grades can be used as "L" grades, as long as their carbon content complies with the carbon content limitation of "L" grades.

It is an increasing practice to have double certified products especially in plates, sheets, pipes and bars. These materials fully comply with 304 / 304L or 316 / 316L grades. While the use of double certified products is used for "L" grades, there is no such application for "H" grades. If an application "H" requires quality, it must be specifically mentioned at the order stage. Standard grades can also be used instead of "H" grades, as long as the carbon content complies with the requirements of "H" grades. Microstructure grain size may be covered by extra examinations. It must specify the material and certificate as "standard", otherwise this material is produced in "H" quality. Details of the test certificate will comply with the requirements of quality.

"H" grades can be used as standard grades 1 as long as their carbon content does not exceed 0.080% and nitrogen content is maximum 0.10%. It usually meets this, but its certificate still needs to be checked.