Austenitic Stainless Steel

Austenitic stainless steel is a specific type of stainless steel alloy. Stainless steels may be classified by their crystalline structure into four main types: austenitic, ferritic, martensitic and duplex stainless steel.

[1] Austenitic stainless steels possess austenite as their primary crystalline structure (face centered cubic).

This austenite crystalline structure is achieved by sufficient additions of the austenite stabilizing elements nickel, manganese and nitrogen. Due to their crystalline structure, austenitic steels are not hardenable by heat treatment and are essentially non-magnetic.

There are two subgroups of austenitic stainless steel. 300 series stainless steels achieve their austenitic structure primarily by a nickel addition while 200 series stainless steels substitute manganese and nitrogen for nickel, though there is still a small nickel content.

300 series stainless steels are the larger subgroup. The most common austenitic stainless steel and most common of all stainless steel is Type SS 304, also known as 18/8 or A2.

Type SS 304 is extensively used in such items as, cookware, cutlery, and kitchen equipment.

Type SS 316 is the next most common austenitic stainless steel. Some 300 series, such as Type SS 316, also contain some molybdenum to promote resistance to acids and increase resistance to localized attack (e.g. pitting and crevice corrosion). The higher nitrogen addition in 200 series gives them higher mechanical strength than 300 series

Other notable austenitic stainless steels are Type 309 and 310, which are utilized in high temperature applications greater than 800°C.

Alloy 20 (Carpenter 20) is an austenitic stainless steel possessing excellent resistance to hot sulfuric acid and many other aggressive environments which would readily attack type 316 stainless.

This alloy exhibits superior resistance to stress-corrosion cracking in boiling 20–40% sulfuric acid. Alloy 20 has excellent mechanical properties and the presence of niobium in the alloy minimizes the precipitation of carbides during welding.

Austenitic stainless steel can be tested by nondestructive testing using the dye penetrant inspection method but not the magnetic particle inspection method. Eddy-current testing may also be used.

Austenitic Stainless Steel – Grade SS 316 (UNS S31600)

SS316 or AISI 316 – Austenitic Stainless Steel

Chemical Formula :
Fe, <0.03% C, 16-18.5% Cr, 10-14% Ni, 2-3% Mo, <2% Mn, <1% Si, <0.045% P, <0.03% S

Grade 316 is the standard molybdenum-bearing grade, second in importance to 304 amongst the austenitic stainless steels.

The molybdenum gives 316 better overall corrosion resistant properties than Grade 304, particularly higher resistance to pitting and crevice corrosion in chloride environments.

It has excellent forming and welding characteristics. It is readily brake or roll formed into a variety of parts for applications in the industrial, architectural, and transportation fields.

Grade 316 also has outstanding welding characteristics. Post-weld annealing is not required when welding thin sections.

We know the various names of stainless steel that are included in the austenitic grade category, namely: AISI / SUS 201, 202, 301, 302, 303, 304, 305, 309, 310, 316, 317, 327, 347. next is the difference of SS 316 and SS 316L and 316H.

What is the difference between 316, 316L and 316H ?

Grade 316L, the low carbon version of 316 ( L=Low Carbon. ) and is immune from sensitisation (grain boundary carbide precipitation).

Thus it is extensively used in heavy gauge welded components (over about 6mm).

Grade 316H, with its higher carbon content has application at elevated temperatures, as does stabilised grade 316Ti.

The austenitic structure also gives these grades excellent toughness, even down to cryogenic temperatures.

Composition ranges for 316 grade of stainless steels.

The following is the chemical composition between SS 316, 316L and 316H

Grade		C	Mn	Si	P	S	Cr	Mo	Ni	N
316	Min	–	–	–	0	–	16.0	2.00	10.0	–
	Max	0.08	2.0	0.75	0.045	0.03	18.0	3.00	14.0	0.10
316L	Min	–	–	–	–	–	16.0	2.00	10.0	–
	Max	0.03	2.0	0.75	0.045	0.03	18.0	3.00	14.0	0.10
316H	Min	0.04	0.04	0	–	–	16.0	2.00	10.0	–
	max	0.10	0.10	0.75	0.045	0.03	18.0	3.00	14.0	–

Mechanical properties of 316 grade stainless steels.

The following are mechanical properties (tensile strength, yield strength, elongation and hardness) of stainless steel SS 316, 316 L and 316 H

Grade	Tensile Str (MPa) min	Yield Str 0.2% Proof (MPa) min	Elong (% in 50 mm) min	Hardness
				Rockwell B (HR B) max	Brinell (HB) max
316	515	205	40	95	217
316L	485	170	40	95	217
316H	515	205	40	95	217

Note: 316H also has a requirement for a grain size of ASTM no. 7 or coarser.

 Typical physical properties for 316 grade stainless steels.

Grade	Density(kg/m3)	Elastic Modulus (GPa)	Mean Co-eff of Thermal Expansion (µm/m/°C)			Thermal Conductivity (W/m.K)		Specific Heat 0-100 °C (J/kg.K)	Elec Resistivity (nΩ.m)
			0-100 °C	0-315 °C	0-538 °C	At 100 °C	At 500 °C
316/L/H	8000	193	15.9	16.2	17.5	16.3	21.5	500	740

Grade specifications for 316 grade stainless steels.

Grade	UNS No	Old British		Euronorm		Swedish SS	Japanese JIS
		BS	En	No	Name
316	S31600	316S31	58H, 58J	1.4401	X5CrNiMo17-12-2	2347	SUS 316
316L	S31603	316S11	–	1.4404	X2CrNiMo17-12-2	2348	SUS 316L
316H	S31609	316S51	–	–	–	–	–

Note: These comparisons are approximate only. The list is intended as a comparison of functionally similar materials not as a schedule of contractual equivalents. If exact equivalents are needed original specifications must be consulted.

Possible alternative grades to 316 stainless steel.

Grade	Why it might be chosen instead of 316?
316Ti	Better resistance to temperatures of around 600-900 °C is needed.
316N	Higher strength than standard 316.
317L	Higher resistance to chlorides than 316L, but with similar resistance to stress corrosion cracking.
904L	Much higher resistance to chlorides at elevated temperatures, with good formability
2205	Much higher resistance to chlorides at elevated temperatures, and higher strength than 316

Corrosion Resistance

Excellent in a range of atmospheric environments and many corrosive media – generally more resistant than 304. Subject to pitting and crevice corrosion in warm chloride environments, and to stress corrosion cracking above about 60 °C. Considered resistant to potable water with up to about 1000 mg/L chlorides at ambient temperatures, reducing to about 500 mg/L at 60 °C.

316 is usually regarded as the standard “marine grade stainless steel”, but it is not resistant to warm sea water. In many marine environments 316 does exhibit surface corrosion, usually visible as brown staining. This is particularly associated with crevices and rough surface finish.

Heat Resistance

Good oxidation resistance in intermittent service to 870 °C and in continuous service to 925 °C. Continuous use of 316 in the 425-860 °C range is not recommended if subsequent aqueous corrosion resistance is important. Grade 316L is more resistant to carbide precipitation and can be used in the above temperature range. Grade 316H has higher strength at elevated temperatures and is sometimes used for structural and pressure-containing applications at temperatures above about 500 °C.

Heat Treatment

Solution Treatment (Annealing) – Heat to 1010-1120 °C and cool rapidly. These grades cannot be hardened by thermal treatment.

Welding

Excellent weldability by all standard fusion methods, both with and without filler metals. AS 1554.6 pre-qualifies welding of 316 with Grade 316 and 316L with Grade 316L rods or electrodes (or their high silicon equivalents). Heavy welded sections in Grade 316 require post-weld annealing for maximum corrosion resistance. This is not required for 316L. Grade 316Ti may also be used as an alternative to 316 for heavy section welding.

Machining

A “Ugima” improved machinability version of grade 316 is available in round and hollow bar products. This machines significantly better than standard 316 or 316L, giving higher machining rates and lower tool wear in many operations.

Dual Certification

It is common for 316 and 316L to be stocked in “Dual Certified” form – mainly in plate and pipe. These items have chemical and mechanical properties complying with both 316 and 316L specifications. Such dual certified product does not meet 316H specification and may be unacceptable for high temperature applications.

Applications

• Food preparation equipment particularly in chloride environments.
• Laboratory benches & equipment.
• Coastal architectural panelling, railings & trim.
• Boat fittings.
• Chemical containers, including for transport.
• Heat Exchangers.
• Woven or welded screens for mining, quarrying & water filtration.
• Threaded fasteners.
• Springs.

Typical applications include:

Austenitic Stainless Steel – SS 316 Equivalent Grade

References:

1.  The International Nickel Company (1974). “Standard Wrought Austenitic Stainless Steels”. Nickel Institute.
2. ^ “Stainless Steel”. Encyclopaedia Britannica.
3. ^ American Iron and Steel Institute. “Design Guidelines for the Selection and Use of Stainless Steels”. Nickel Institute.
4. Atlas Steels Australia