Dual grade stainless steel 304 /304L, 316/316L

Austenitic stainless steels are the most widely used stainless steels, accounting for about 75% of the total stainless steel consumption. The rapid development of the chemical industry and petrochemical industry has put forward higher requirements for the corrosion resistance and strength of stainless steel. For example, the 304/304L dual grades stainless steel means it has lower carbon content, that’s less than 0.03%, meeting the 304L grades, while its yield and tensile strength are higher than the lower limit of 304 stainless steel, the stainless steel can be defined as 304/304L dual grades stainless steel, that is, its chemical composition meets that of 304L, and mechanical properties to meet the requirements of 304 stainless steel. Similarly, a stainless steel sheet can be 304/304H dual certified because it has enough carbon content to meet the 304H (minimum 0.040%) requirement and also meets the 304H grain size and strength requirements, there are 316/316L and other dual grades of stainless steel.

The most important is the difference in carbon and the resulting strength. Carbon is an effective austenitic stabilizing element and can be considered as an impurity or an alloying element that improves the strength of stainless steel, especially at high temperatures. The carbon content in most austenitic stainless steels is below 0.02% ~ 0.04%. In order to have good corrosion resistance after welding, the carbon content of low carbon grade stainless steel is controlled below 0.030%. In order to improve the high-temperature strength, the high carbon or “H” grade carbon content is maintained at 0.04% or slightly higher.

The smaller carbon atoms in the face-centered cubic structure are in the lattice gaps between the larger Cr, Ni and Mo atoms, which limit the dislocation motion, hinder the ductility deformation and strengthen the stainless steel. Under the condition of rising temperature such as in the welding process, carbon has a strong tendency to precipitate chromium in stainless steel matrix with chrome-rich carbide, and the second phase tends to precipitate at grain boundary rather than grain center, so chromium carbide is easy to form at the grain boundary.

Chromium is a necessary element for enhancing the corrosion resistance of stainless steel, but chromium carbide is removed from the stainless steel matrix, so the corrosion resistance here is worse than the rest of the stainless steel matrix. Increasing the carbon content can extend the temperature range, so that the time of sensitization or corrosion resistance loss is shortened, reducing the carbon content can delay or completely avoid the formation of carbide in welding. Low carbon grades such as 304L and 316L carbon content less than 0.030%, most of the higher alloyed Austenite grades such as 6%Mo stainless steel carbon content is less than 0.020%. To compensate for the decrease in strength due to the decrease in carbon content, another interstitial element nitrogen is sometimes added to strengthen the stainless steel.

Dual-grade stainless steel has both the high strength of conventional stainless steel and the corrosion resistance of ultra-low carbon stainless steel. It can solve the problem of weak welding joint performance of most Austenitic stainless steel, has been widely used in low-temperature LNG receiving station equipment and large diameter pipeline. The price of dual-grade stainless steel is basically the same as ultra-low carbon stainless steel. Now several Chinese steel mills can supply the grades for mature market, any interested, please contact us.

 

What’s Super 304H steel?

With the development of ultra-supercritical units, the high-temperature strength of traditional 18-8 Austenitic stainless steels (such as TP304H steel) has been unable to meet their needs with steam parameters of 600℃. For this reason, Japan Sumitomo Metal Corporation has developed new materials for the boiler heating surface pipeline of the unit, like TP347HFG steel, SUPER304H steel and HR3C steel. Super 304H steel is a new type of 18-8 steel, mainly used in the manufacture of superheater and reheater of ultra-supercritical boilers whose metal wall temperature does not exceed 700 ℃. At present, Shasqida Mannesmann (formerly DMV Company) in Germany also produces similar steel tubes, with a grade DMV 304HCU.

Super304H steel is the steel by reducing the content of Mn, Si, Cr and Ni-based on TP304H steel, which adding 2.5% ~ 3.5% Cu and 0.30%~0.60% of Nb and 0.05%~0.12% of N, so that to produce the diffusion precipitation phase and copper-rich strengthened phase in service, occurs precipitation strengthening with NbC(N), NbCrN and M23C6, which greatly increases the allowable stress at service temperature, and the allowable stress at 600 ~ 650℃ is 30% higher than that of TP347H steel. The steam oxidation resistance of the steel is comparable to that of TP347HFG steel and significantly better than that of TP321H steel. It has been listed in ASME Code Case 2328-1, ASTM A-213 Standard, the number is S30432.

 

The Chemical Composition of Super 304H

C Si Mn P S Cr Ni N Al B Nb Cu V Mo
0.08 0.21 0.79 0.03 0.001 18.42 8.66 0.11 0.007 0.004 0.5 2.77 0.04 0.35

 

The Mechanical Property of Super 304H

Yield strength, Mpa Tensile strength, Mpa Elongation, %
360/350 640/645 58/60

 

Due to the high steam parameters of ultra-supercritical units, the oxidation resistance of steel used in high-temperature pressure parts of power plants becomes very important. Generally, the inner wall of the super 304H steel pipe is shot blasting to improve the anti-steam oxidation performance. A 30μm thickness shot blast layer was formed on the inner surface of the steel tube and its microstructure was refined compared with that of the non-shot peening steel tube. After the steam oxidation test at 650℃ and 600h, the oxide layer thickness of the steel tube treated by the shot blast is thinner and denser, and the steam oxidation resistance of the steel tube is improved. Currently, several leading steel mills in China have produced a similar grade 10CrL8Ni9NbCu3Bn, specified in GB 5310-2008, which is currently used in several ultra-supercritical unit projects in China.

Is 304 stainless steel magnetic?

Ordinary consumers have some misunderstandings about stainless steel, they think that the magnetic stainless steel is not qualified 304 stainless steel. As we know, according to the structure under room temperature,stainless steel can be divided into Austenite such as 201, 304, 321, 316, 310, Martensite or Ferric such as 430, 420, 410. Austenites are non-magnetic or weakly magnetic and Martensite or ferrite are magnetic. 304 is a representative grade of the austenitic stainless steel, it has excellent workability, weldability and corrosion resistance, account for 60% of the world consumption of stainless steel, generally, it is no magnetic, but sometimes it is magnetic or weak magnetism caused by smelting chemical composition fluctuations or processing, but we cannot think this is fake or substandard, what reason is this?

304 is metastable stainless steel, is a single austenite structure after annealing state, with no magnetic. Smelting composition segregation or improper heat treatment will produce a small amount of martensite or ferrite structure, so with a weak magnetic. In addition, after the cold processing deformation (such as stamping, stretching, rolling, etc.), part of the austenite structure also underwent phase change (general mutagenesis into martensite) and with magnetic.

For example, in the same batch of steel strips, the outer diameter of 76mm steel pipe has no obvious magnetic while the outer diameter of 9.5mm steel pipe has obvious magnetic. The magnetic properties of the square rectangular tube are more obvious because the cold bending deformation is greater than that of the round tube, especially in the bending part.

Most of the water sink is made of 304 stainless steel. Many consumers judge it is made of 304-grade stainless steel according to whether the water tank is magnetic or not. At present, there are many kinds of processing technology for the sink, such as welding forming, integral tensile forming, etc., if used 304 material welding forming, is generally annealed after the plate processing, will not be magnetic or weakly magnetic (because of the surface treatment of the sink); One of the water tank drawing molding needs to go through several stretching, general annealing and then stretching (annealing increases the cost, and 304 is not necessary to anneal again), it will be magnetic, that is a very normal phenomenon.

304 stainless steel VS 403 stainless steel

Grades 304 and 430 are commonly used stainless steel materials. 304 stainless steel is a general type of chromium-nickel austenitic stainless steel, the density of 7.93 g/cm3, also known as 18/8 stainless steel, is 300 series of stainless steel is the most commonly used steel. It can withstand high temperature 800℃, has good processing performance and toughness, widely used in the requirements of good comprehensive performance (corrosion resistance and molding) equipment and parts. 304L is a low-carbon version of 304, which does not require post-weld annealing, so it is widely used for thick gauge parts (approx. 5mm and above). The higher carbon content of 304H can be used at high temperatures. The annealed austenite structure also gives these grades excellent toughness, even at low freezing temperatures.

Low carbon high chromium 430 is one of the most common ferritic stainless steels, has good corrosion resistance, also known as 18/0 or 18-0, is one of the 400 series of stainless steels. It can be made slightly strengthened by cold working, but the low-temperature toughness is poor, and generally can not be hardened by heat treatment. Its thermal conductivity is better than austenite, the coefficient of thermal expansion is smaller than austenite, heat resistance fatigue, the addition of stabilizing element titanium makes the welding seam part of the mechanical property is good, can be used for building decoration, fuel burner parts, household appliances, household appliances parts. 430F is a kind of steel with free cutting performance on 430 steel, mainly used for automatic lathes, bolts and nuts, etc. 430LX adds Ti or Nb in 430 steel, reduces the content of C, and improves the processing performance and welding performance. It is mainly used for hot water tanks, heating water systems, sanitary appliances, household durable appliances, bicycle flywheels, etc.

 

According to ASTM A240- Specifications for chromium and chromium-nickel stainless steel plates, sheets and strips for pressure vessels and general purposes, 430 stainless steel shall contain less than 0.12% carbon, between 16-18% chromium, and less than 0.75% nickel, the difference between 304 and 430 as shown in the table below:

Chemical composition comparison 

UNS C Mn P S Si Cr Ni Mo
S30400 0.07 2.00 0.045 0.03 0.75 17.5-19.5 8.0-10.5 /
S43000 0.12 1,00 0.04 0.03 1.00 16.0-18.0 0.75 /

 

Mechanical property comparison

Grades Yield strength, Mpa Tensile strength, Mpa Elongation in 2 /50mm, min, % Hardness, HBW
304 205 515 40 183
403 205 450 22 201

 

To sum up, they differ mainly in the following items:

  • Corrosion resistance: The corrosion resistance of 304 stainless steel is better than 430. Because 430 stainless steel contains 16.00-18.00% chromium, basically does not contain nickel, 304 stainless steel contains more chromium and nickel;
  • Stability: 430 stainless steel is ferrite form, 304 stainless steel is austenite, more stable than 430 stainless steel;
  • Toughness: The toughness of 304 is higher than 430 stainless steel;
  • Thermal conductivity: The thermal conductivity of ferrite 430 stainless steel is like 304 stainless steel;
  • Mechanical properties: 430 stainless steel welding seam mechanical properties than 304 stainless steel is better because of the addition of stable chemical element titanium.

302HQ VS 304 Stainless Steel

302HQ stainless steel is a standard material specifically used in the manufacture of self-tapping screws and light mechanical screws. It is also used in bolts, setting screws, rivets, and special fasteners. The name 302HQ is not standardized. The ASTM lists it as UNS S30430, which also includes “XM-7”, “304CU”, and “304HQ”. It has now completely replaced 384 and 305 steel for cold heading purpose. ISO 3506, Standard specification for stainless steel fasteners, 302HQ as an eligible component for class “A2” fasteners; It is commonly used to manufacture fasteners in the A2-70 and A2-80 strength. The stable austenitic structure enables 302HQ to be non-magnetic even after extensive cold working and to maintain excellent toughness at temperatures as low as freezing. Compared with 304 stainless steel, the addition of 3% copper in 302HQ can significantly reduce the cold work hardening rate. The chemical composition and physical properties are shown below:

 

Equivalent Material

Grades UNS No DIN EN JIS
302HQ S30430 1.4567 X3CrNiCu18-9-4 SUSXM7

 

Chemical Composition (ASTM A493 S30430)

Grades C Mn Si P S Cr Mo Ni Cu
302HQ 0.03 2.00 1.00 0.045 0.03 17.0-19.0 / 8.0-10.0 3.0-4.0

 

Mechanical Property

302HQ tensile strength: Annealing: 605, Mild drawing: 660

Density: 7900kg/㎡

Elasticity modulus:193Gpa

Average coefficient of thermal expansion: 0-100℃ (um/m/℃) 17.2; 0-315℃ (um/m/℃); 0-538 ℃ (18.8)

Thermal conductivity: 100℃ (W/ M. K) 16.3; 500℃ (W/ M. K) 21.5

Specific heat: 0-100℃ (J/ kg.K) 500;

Resistance: 720

 

Corrosion resistance

Its corrosion resistance is equivalent to or superior to 304 stainless steel. Pitting and crevice corrosion is easy to occur in the warm chloride environment, and stress corrosion cracking is sensitive when the temperature is higher than about 50°C. 302HQ can withstand about 200mg/L chloride in drinking water at room temperature and 150mg/L at 60℃.

 

Heat Resistant Performance

Good oxidation resistance, intermittent use temperature up to 870°C, continuous use temperature up to 925°C. Because of the low carbon content of 302HQ, it is safe for continuous use (no carbide precipitation) ranges from 425 to 860°C.

 

Heat Treatment

Solution treatment (annealing) is heated to 1010-1120°C and rapidly cooled. Heat treatment will not harden it.

 

Weldability

Excellent weldability, all standard fusion welding methods (whether or not they contain filler metal) can be used. Use a 308L electrode. Welding is generally not required except in the manufacture of stud welded fasteners, where resistance butt welding is used to join wires together.

 

Processing 

The 302HQ is rarely machined. The grade has a very low sulfur content, which helps its formability but reduces its machinability. The Improved 302HQ (UGIMA 4567) has very high machinability, slightly higher sulfur content, and is also calcium treated for use requiring extensive cold forming and machining operations on the 18/8 steel.

 

Cold Work Hardening

302HQ is the lowest work hardening rate among the common grades of austenitic stainless steels. According to the wire drawing data, the tensile strength increases by 8MPa when the cold-working area decreases by 1%). Even after extensive cold work, the brand remains essentially unresponsive to magnets. Some high strength cold heading fasteners require a slightly higher work hardening rate, so 304 or 304L (or special grade 304M) should be used instead of 302HQ; The work hardening rate of these grades is about 10-12.5MPa.

 

Typical Applications

All harsh cold heading applications, including self-tapping screws, roof bolts, mechanical screws, bolts, set screws, blind rivets, etc.

Stainless steel 321 VS 347

The property of 321 stainless steel and 347 stainless steel is similar in most cases, the 321 stainless steel is a kind of titanium – stabilization of 18/8 austenitic stainless steel (304), a small amount of titanium makes it in carbide precipitation temperature range, that’s 425-850℃, not appear intergranular corrosion after heating, with good strength, resistance to oxidation peeling and aqueous corrosion resistance.

The 321H is a high-carbon version of the 321 with higher high-temperature strength and is primarily used for high-temperature applications around 900°C. The disadvantage of 321 is that titanium has poor welding arc transition, so it cannot be used as welding material, while 347 containing niobium also plays the role of carbide stabilization, and can also be transferred through welding arc. 347 is standard welding material for 321 stainless steel welding and occasionally used as the base metal. Let’s see their chemical and mechanical comparison below:

 

Chemical composition comparison

Grades C Mn Si P S Cr Ni Mo N Other
321 0.08 2.00 0.75 0.045 0.03 17.0-19.0 9.0-12.0 / 0.1 Ti=5(C+N)0.7
347 0.08 2.00 0.75 0.045 0.03 17.0-19.0 9.0-13.0 / / Nb=10(C+N)1.0

We can see that the difference between them is the addition of  Ti and Nb. Due to the addition of stabilized element titanium, 321 can resist the formation of chromium carbide at 426℃~815℃, so it has excellent intergranular corrosion resistance and high-temperature performance and has higher creep and stress fracture properties than 304 and 304L. In addition, 321 also has good low-temperature toughness and excellent formability and welding characteristics, without annealing after welding.

347 Stainless steel is a niobium-containing Austenitic stainless steel and 347H is its high carbon version. 347 can be seen as a niobium-adding version based on 304. Nb, a rare earth element, has a similar effect to titanium in refining grains,  can resist intergranular corrosion and promoting aging hardening.

 

Physical property comparison

Grades Tensile strength, Mpa Yield strength, Mpa Elongation(50mm) Hardness, HB
321 515 205 40 217
347 515 205 40 201

 

Typical applications

347&347H stainless steel has better high-temperature performance than 304 and 321. It is widely used in aviation, petrochemical, food, papermaking and other industries, such as exhaust pipe and branch pipe of aero engine, hot gas pipe of turbine compressor and parts working under low load and temperature not exceeding 850℃.

The addition of titanium to the 321 makes it more suitable for where need high temperature and good corrosion resisting applications. It is suitable for 304 sensitized and 304L applications with insufficient high-temperature strength. Typical applications include thermal expansion joints, bellows, aircraft exhaust system components, heating element sleeves, furnace components, and heat exchangers.