304 and 321, 316L stainless steel difference and welding characteristics

304 Stainless steel Properties

304H, 304L and 304 stainless steels are several deformation varieties of 18% Cr and 8% Ni austenitic stainless steels. They are the most commonly used and common stainless steel products in the 304 stainless steel family.

These stainless steel varieties have one or more of the following properties and are widely used. Properties include: corrosion resistance, pollution prevention, oxidation resistance, easy processing, better shape, beautiful appearance, easy to clean, high strength, low weight, good strength and toughness in low temperature environment. Almost all of them have corrosion resistance and better workability.

304,304 L, 304H is also known as 18Cr-8Ni stainless steel, there are various forms, plate, roll, piece, strip. Commonly used in manufacturing equipment, applications include but are not limited to: food, medical, hygiene, refrigeration, pressure vessels, etc.

It is through argon-oxygen decarbonization technology that stainless steel achieves low carbon at low cost, providing favorable conditions for 304 to become the most commonly used standard stainless steel.

304L is mainly used for welding, because they often operate in various corrosive environments.

304H stainless steel is an improvement of 304 stainless steel, its carbon content between 0.04-0.10, for exposed to the temperature of 800°F above the parts, the selection of 304H helps to improve the strength of stainless steel at high temperature.

Austenitic 304 stainless steel, good corrosion resistance in suitable oxidation and reduction environments. They are often used for processing, handling food and beverage, heat exchangers, pipes, oil tanks, dairy equipment and appliances.

The chromium content of these stainless steel accounts for 18-19%, with strong oxidation resistance. The following table shows the oxidation rate of 304 stainless steel in dilute nitric acid environment: The oxidation rate of stainless steel in dilute nitric acid environment

304, 304L, and 304H are also resistant to moderate organic acids (such as acetic acid) and reducing acids (such as phosphoric acid). 18-8 stainless steel has 9-11% nickel content and is resistant to moderately reducing environments. But in more reductive reducing environments (such as boiling dilute hydrochloric acid and sulfuric acid), corrosion is too strong.

In some cases, low-carbon 304L stainless steel has a lower corrosion rate than high-carbon 304 stainless steel. Data from formic acid, sulfamic acid, and sodium hydroxide support this. In other times 304, 304L and 304H have the same performance in most corrosive environments. It is important to note that 304L stainless steel is preferred in the environment sufficient to cause welding and heat affected zone corrosion, because its low carbon content helps to resist corrosion between particles.

Intergranular corrosion

18-8 austenitic stainless steel at 800°F -- 1500°F (427°C to 816°C) may result in precipitation of chromium carbide at grain boundaries. It is prone to intergranular corrosion in harsh environments. The carbon composition of 304 stainless steel leads to sensitization during the thermal state of gas welding and heat-affected zone welding.

Stainless steel intergranular corrosion stress corrosion cracking

304, 304L, 304H stainless steel is the most prone to stress corrosion cracking in austenitic stainless steel, because their nickel content is relatively low. The conditions that cause stress corrosion cracking are:

(1) Presence of halide ions (usually chloride)

(2) Residual tension

(3) Temperature exceeding 120°F (49°C)

In the process of stainless steel forming cold deformation, stretching tube plate, welding operation can produce stress. The stress can be reduced by annealing and stress relief heat treatment after cold deformation, thus reducing the possibility of stress corrosion cracking of halide. In the environment that may cause intergranular corrosion, low temperature annealing operation, the best choice of low carbon 304L stainless steel material.

Stainless steel 18-8 Stainless steel can be used well in fresh water with low chloride content. The corrosion resistance limit of 18-8 stainless steel is 100ppm chloride when there is a gap. High chloride content may cause gap corrosion and spot corrosion. At low PH or high temperature, stainless steel with high molybdenum content, such as 316, should be used. 18-8 stainless steel also cannot be used in Marine environments.

Physical properties of stainless steel

Density: 0.285lb /in3 (7.90g /cm3)

Modulus of tensile elasticity: 29 x 106 psi (200 GPa) Linear thermal expansion coefficient: Stainless steel linear thermal expansion Heat conduction:

The total conductivity of stainless steel heat conduction metal is determined by other factors besides the thermal conductivity of the metal. 18-8 stainless steel has the ability to keep the surface clean, and compared to other metals with high heat conductivity, 18-8 stainless steel has better heat conductivity.

Specific thermal permeability of stainless steel: 18-8 stainless steel is non-magnetic in the annealed state, and the permeability is generally lower than 1.02 under the condition of 200H. The permeability will vary depending on the composition of the metal. By cold working, the permeability can be improved.

Mechanical properties of stainless steel Mechanical properties at room temperature

The minimum mechanical properties required for 304 and 304L annealed austenitic stainless steel plates, ASTM Standard A240, ASME Standard SA-240, are shown in the following table:

Mechanical properties of stainless steel properties at low and high temperatures

The short-term tensile properties under low temperature and temperature rise are shown in the table below. Stress cracking should be considered when the temperature reaches 1000°F (538°C) or above. Stress cracking data are also shown in the table below.

Low temperature performance of stainless steel

Annealed austenitic stainless steel can maintain high impact resistance even at low temperatures, coupled with low temperature hardness and workability properties, so it is used for liquefied natural gas and other low temperature operations. The data of Xia's V-shaped impact experiment are shown in the table below:

Stainless steel impact resistance

The maximum stress of metal material under infinite multiple alternating loads without failure is called fatigue strength or fatigue limit. The fatigue strength of austenitic stainless steel is generally 35% of the tensile strength. In the actual operation, the fatigue strength will also be affected by other factors, such as: increase the smoothness of the surface, can increase the fatigue strength, the corrosion of the operating environment, reduce the fatigue strength.

Austenitic stainless steel is considered the most easily welded stainless steel and can be welded with all fusions or resistance welded. 304 and 304L are typical austenitic stainless steel.

Two factors should be considered in the production of welding contacts of austenitic stainless steel: 1) to maintain corrosion resistance and 2) to avoid cracking.

As the material is welded, a temperature ladder is formed, from the melting temperature of the molten pool to the surrounding temperature slightly further away from the weld point. The higher the carbon content of the material to be welded, the more easily the welding thermal cycle leads to chromium carbide precipitation, which has an impact on the corrosion resistance of the material. In order to maintain the corrosion resistance of the material at the best level, so in the welded state of operation, should choose low carbon material (304L). An alternative is to dissolve chrome carbide with full annealing, restoring a high level of corrosion resistance to materials with standard carbon content.

Cracks are more likely to form in the welding operation of completely austenitic metals. Therefore, 304 and 304L stainless steel added a small amount of ferrite, reduce the crack sensitivity of the material, to achieve the role of recuring.

When welding 18-8 austenitic stainless steel to carbon steel, 309 stainless steel (23%-13.5% nickel) or nickel-based solder is usually used.

Stainless steel heat treatment

Austenitic stainless steel by heat treatment can ** cold forming produces side effects and dissolution precipitation of chromium carbide. The best heat treatment to meet these two requirements is solid annealing in the temperature range of 1850°F to 2050°F (1010°C to 1121°C). Cooling down from the annealing temperature of 1500-800°F (816° C-427 °C) should be sufficient to avoid reprecipitation of chromium carbide.

These materials cannot be hardened by heat treatment.

Stainless steel cleaning

No matter how corrosive, stainless steel in the process of processing and use, to keep its surface clean.

In the welding process using inert gas processing, welding process formed rust and slag through the stainless steel brush. Ordinary carbon steel brushes leave carbon steel particles on the surface of stainless steel. These particles can eventually cause surface rust. In strict cases, the welding area is treated with a derusting solution (such as a mixture of nitric acid and hydrofluoric acid) to wash away the rust and slag formed during the welding process.

Light industrial materials require less maintenance, and only sheltered areas sometimes need to be cleaned with pressurized water. Heavy industry recommends frequent cleaning to remove accumulated dust, which may eventually cause corrosion and damage the surface appearance of stainless steel.

Stubborn stains and sediment can be scrubbed with detergent and fiber brush, sponge, stainless steel velvet. Stainless steel velvet will leave long scratches on smooth stainless steel surfaces.

A lot of stainless steel should be cleaned and cleaned regularly. Equipment is usually cleaned with special caustic soda, organic solvent, and acid solution (into phosphoric acid or sulfuric acid). Strong reducing acids (such as hydrofluoric or hydrochloric acid) may damage stainless steel.

After solution cleaning, rinse stainless steel thoroughly with clean water.

Proper design helps with cleaning. With round erasing, inner rounded corners, seamless equipment, conducive to cleaning and surface polishing.

Alloy 321 (UNS S32100) is a stable stainless steel plate whose main benefit is excellent intercrystalline corrosion resistance when exposed to silicon carbide precipitation temperatures ranging from 800 to 1500°F (427 to 816°C). By adding titanium, the alloy 321 stainless steel plate is stabilized to prevent chromium carbide formation.

Alloy 321 stainless steel plate is also favorable for high temperature service due to its good mechanical properties. Alloy 321 stainless steel plate has higher creep and stress fracture properties than alloy 304, especially alloy 304L, and may also be considered for sensitivity and intergranular corrosion problems.

Alloy 321 (UNS S32100) is a titanium-stabilized austenitic stainless steel plate with good general corrosion resistance. It has excellent resistance to intergranular corrosion in the precipitation temperature range of 800-1500 °F (427-816 °C). The alloy resists oxidation to 1500°F (816 ° C) and has higher creep and stress fracture properties than alloys 304 and 304L. It also has good low temperature toughness.

Alloy 321H (UNS S 32109) stainless steel plate is a higher carbon (0.04-0.10) version of the alloy. It was developed to improve creep resistance and has higher strength at temperatures above 1000°F (537°C). In most cases, the carbon content of the plate can be two-factor certified.

Alloy 321 stainless steel plate can not be hardened after heat treatment, only cold working. It can be easily welded and machined to standard shop manufacturing practices.

Alloy 321 stainless steel plate has good general corrosion resistance comparable to 304. It was developed for use in the precipitation range of 1800-1500°F (427-816°C) for silicon carbide, where unstable alloys such as 304 to intergranular attack.

The alloy can be used at moderate temperatures for most diluted organic acids, at lower temperatures for pure phosphoric acid, and at high temperatures for up to 10% diluted solutions. Alloy 321 in hydrocarbon service resists stress corrosion cracking with polysulphuric acid. It can also be used in chlorinated or fluorine-free solutions at moderate temperatures.

Alloy 321 stainless steel plates do not perform well in chloride solutions, even in small concentrations, and cannot be used in sulfuric acid. Alloy 321 stainless steel plates can be easily welded and machined through standard shop manufacturing practices.

The cold working hardening rate of 321 stainless steel plate is lower than that of 410 stainless steel plate, but similar to 304. The following table provides relevant processing data.

Machinability of stainless steel

welding

Among high alloy steels, austenitic stainless steel is considered the most weldable and can be subjected to various fusion and resistance welds. When welding austenitic stainless steel, two important issues should be considered: corrosion protection and avoid cracking. You want to keep the element stable. The use of inert gases requires a certain degree of cleanliness, and avoid the absorption of C from oil or N from air. Steel 321 is more likely to lose Ti than steel 347, while steel 348 May lose Nb.

When welding metal with full austenitic structure, it is easier to crack in the welding process. Therefore, 347, 348 and 321 stainless steels are solidified with a small amount of ferrite to minimize crack sensitivity. Nb stabilized stainless steel is more likely to crack than Ti stabilized stainless steel.

annealing

The annealing temperature range for 321 and 347 steels is 1800 -- 2000°F (928 -- 1093°C). The main purpose of annealing is to soften and obtain high ductility. These steels may also be stress-relieved annealed in the temperature range 800 -- 1500°F (427 -- 816°C) for carbide precipitation without risk of intercrystalline corrosion. Strain relief annealing for only a few hours in the temperature range 800-1500 °F (427 -- 816°C) does not significantly reduce general corrosion resistance. Of course, long time heating in this temperature range will indeed reduce the general corrosion resistance to a certain extent. However, as emphasized, annealing in the 800 -- 1500°F (427 -- 816°C) temperature range does not produce sensitivity to intergranular erosion.

For maximum ductility, annealing in the higher temperature range of 1800 -- 2000°F (928 -- 1093°C) is recommended.

When using Cr-Ni stainless steel processing equipment, stable steel should be used to prevent carbide precipitation to the greatest extent. It is necessary to identify the difference in stability between Nb, an element that generates carbides more readily than Ti. Therefore, the stability and protection of 321 steel may not be obvious.

If maximum corrosion resistance of 321 steel is required, corrosion improvement measures called stable annealing must be used. Heat the material to 1550-1650DF (843-899DC) for up to 5 hours, depending on the thickness. This range is above the formation temperature of chromium carbide and is high enough to decompose and solid dissolve all carbides formed before. In addition, this temperature causes Ti and C to combine to form harmless carbides. The result is that Cr will be reduced to a solid solution and C will have to combine with Ti to form a harmless carbide.

If the heat treatment is performed in an oxidizing atmosphere, the oxides are removed after annealing in a mixture of nitric acid and hydrofluoric acid, which must be thoroughly rinsed from the surface of the material.

clean

Despite its corrosion resistance, stainless steel should be carefully processed and used to maintain its surface condition even under normal service conditions.

Welding using inert gas protection process, welding produced oxide skin and slag with stainless steel wire brush. Ordinary carbon steel brushes leave carbon steel particles on the surface, eventually rusting the surface. For more stringent applications, the weld area is treated with a derusting solution such as a mixture of nitric acid and hydrofluoric acid to remove the oxide film color and must be immediately flushed with water.

When materials are used inland, in light industrial or milder conditions, minimal maintenance is required. Only sheltered areas are occasionally flushed with pressurized water. In the ocean or heavy industrial areas, water can often be used to remove salt and dirt deposits that damage the appearance of stainless steel.

Stubborn spots and deposits, like burned food, can be washed with a nonabrasive cleaner and a fiber brush, sponge, or stainless steel cotton. But stainless steel cotton leaves a lasting mark on a smooth stainless steel surface.

Stainless steel needs regular cleaning and cleaning in many applications. The equipment is cleaned with a special sodium hydroxide, organic solvent or acid solution such as phosphoric acid or sulfamic acid (strong reducing acids such as hydrofluoric or hydrochloric acid may be harmful to these stainless steels). The cleaning solution must be drained and the surface of the stainless steel thoroughly bleached with fresh water. If the residual solution comes into contact with the surface of the stainless steel for a long time, it will deteriorate.

Improved design performance contributes to the cleanability of stainless steel. Equipment with few fillets, chamferes and cracks is easier to do cleaning work such as weld sanding and surface polishing.

Welding performance of 316L stainless steel

316L stainless steel belongs to ultra-low carbon pure austenitic stainless steel, good welding performance, low possibility of intergranular corrosion formation, but because of its small thermal conductivity, linear expansion coefficient is large, so the steel welded joint in the cooling process will produce large tensile stress, welding heat input is large, cooling speed is slow and easy to form thermal cracks, corrosion cracking and deformation.

316L stainless steel can be welded with all standard welding methods, in welding according to the use of different, can be used respectively 316Cb, 316L or 309Cb stainless steel filler rod or electrode to weld; In common welding methods, the heat input of MIG and TIG welding is small, and argon flow not only protects the high temperature metal, but also has a certain degree of cooling effect, which increases the crack resistance of the weld and reduces the welding deformation.

The use of 316L stainless steel, generally do not need to do post-welding annealing treatment, austenitic stainless steel after welding generally do not stress relief annealing heat treatment. The reason is that the plasticity and toughness of austenitic stainless steel is very good, without the post-welding stress relief annealing heat treatment to restore its performance; Secondly, 450~850℃ temperature range is the sensitization temperature of austenitic stainless steel. Austenitic stainless steel is heated in this range for a long time, which will make its corrosion resistance decline. If there is ferritic composition in the weld, it may also produce 475℃ brittleness. The post-welding stress relief annealing is just within the temperature zone (except solution treatment and stabilization treatment).

But sometimes special circumstances, also to 316L stainless steel after welding stress relief annealing heat treatment, one is to stabilize the geometric shape of equipment parts, need to eliminate welding residual stress; The other is that the equipment is working in an environment prone to stress corrosion, and the tensile residual stress also needs to be eliminated.

316L stainless steel

304 Weldability

Austenitic stainless steel with 18%Cr-8%Ni stainless steel as the representative, that is, 304 stainless steel is often said, welding processing in principle without pre-welding and post-welding heat treatment. It usually has good welding properties. But the content of nickel and molybdenum is high, so it is easy to produce high temperature cracks during welding. In addition, cross-embrittlement (Fe‐Cr intermetallic compounds) occurs, where ferrites formed in the presence of ferrite forming elements cause low temperature embrittlement, and defects such as decreased corrosion resistance and stress corrosion cracking.

After welding, the mechanical properties of 304 stainless steel welded joints are good, but when there is chromium carbide on the grain boundary in the heat affected zone, it is very easy to form chromium poor layer, and chromium poor layer will lead to intergranular corrosion easily in the process of product use. In order to avoid problems, it is best to use low carbon (C≤0.03%) brand or add titanium, niobium brand. In order to prevent high temperature cracking of welded metals, it is generally considered to be effective to control the δ ferrite in austenite. Generally, it is better to contain more than 5% ferritic composition at room temperature. The main use is corrosion resistant stainless steel, low carbon and stable steel should be selected, but also appropriate post-welding heat treatment; The steel whose main purpose is structural strength should not be subjected to post-welding heat treatment in order to prevent deformation and mutual embrittlement due to carbide precipitation.

Comparison of corrosion resistance between 316L and 304 stainless steel

316L corrosion resistance

316L stainless steel as a molybdenum containing stainless steel, its corrosion resistance is better than 304 stainless steel, manufacturing pulp and paper production equipment has excellent corrosion resistance. And 316 stainless steel is also resistant to the erosion of the ocean and erosive industrial atmosphere. High heat resistance, in the intermittent use below 1600 degrees and in the continuous use below 1700 degrees, 316L stainless steel has good oxidation resistance. In the range of 800-1575 degrees, it is best not to continuously work 316L stainless steel products, but in the temperature range outside the continuous use of 316 stainless steel, it has good heat resistance.

The resistance of 316L stainless steel to carbide precipitation is better than 316 stainless steel, available in the above temperature range. 316L, as the low C series of 316 steel, has the same characteristics as 316 steel, and its resistance to grain boundary corrosion is excellent. It is a product with special requirements for resisting grain boundary corrosion in the use of 316 steel.

304 corrosion resistance

304 stainless steel is a kind of high alloy steel that can resist corrosion in the air or chemical corrosive medium. 304 stainless steel has excellent corrosion resistance and intergranular corrosion resistance. For oxidizing acid, the experimental results are: for concentration ≤65% boiling temperature of nitric acid, 304 stainless steel has a strong corrosion resistance. It also has good corrosion resistance to alkali solution and most organic and inorganic acids.

304 stainless steel rust reasons are mainly several, one is the presence of chloride ions in the use of the environment; Second, stainless steel is not after solution treatment. Alloy elements do not dissolve into the matrix, resulting in low alloy content of basic structure and poor corrosion resistance; The third is that the material, which does not contain titanium or niobium, has an innate tendency to intergranular corrosion. The intergranular corrosion can be reduced by adding titanium and niobium to the stable treatment.

In addition, 316L and 304 stainless steel in the chemical composition of the main difference is that 316L stainless steel containing molybdenum. The thermal strength and creep strength of austenitic stainless steel can be enhanced by adding molybdenum as alloying element. Improve its resistance to pitting and intergranular corrosion.

Molybdenum in reducing and strong oxidizing salt solution can passivate the surface of stainless steel and improve the corrosion resistance to prevent pitting corrosion of steel in chloride solution. The addition of Mo can improve the resistance to reducing acid and pitting corrosion, and reduce the content of carbon can improve the resistance to intergranular corrosion and improve the welding performance.

The addition of molybdenum element can better prevent pitting corrosion. 304 belongs to low carbon stainless steel and 316L belongs to ultra-low carbon stainless steel. However, both 304 and 316L are more sensitive to Cl particles, and 304 has weaker cl- resistance than 306L. Therefore, 316L is usually used in environments with relatively high CL- content.