The role of molybdenum elements in austenitic stainless steel pipe

In general, simple chromium nickel, and chromium manganese nitrogen austenitic stainless steel pipe not only used for rust and oxidation resistance of medium (such as nitric acid, etc.) conditions, the use of molybdenum austenitic stainless steel as the important alloy elements added to the further expansion of the use of steel, the effect of molybdenum is mainly in the reducing medium steel (than - H2SO4, H3PO4, and some organic acid and urea environment) corrosion resistance, and improve the pitting corrosion and crevice corrosion resistance and other properties of the steel.

The chemical identification method of stainless steel By yaang.com

Stainless steel, it is to point to weak corrosive medium such as air, steam, water resistance and acid, alkali, salt and other chemical etching medium corrosion of the steel, also called acid-proof stainless steel. Stainless steel itself because of its corrosion resistance, high temperature resistant, wear resistant features, in our country focus on the development of energy, petrochemical, electric power, mining and other fields will be more active to play its role. Products include stainless steel flanges and stainless steel pipe fittings.

Selection of Stainless Steel for Water Tank Application By yaang.com

Stainless steel grades, such as the 304 (1.4301) or 316 (1.4401) types are generally suitable for storing and handling cold or unheated drinking town's waters. Localised corrosion by crevice or pitting mechanisms is not usually a hazard in properly designed, fabricated and finished tanks handling clean waters of drinking quality.

Nitrogen and Molybdenum Chromium In Stainless Steel By yaang.com

Molybdenum effect
Molybdenum in pure γ-Fe in the maximum solubility of about 3%, ω (C) ≈ 0.25% ~ 0.30%, the maximum solubility in austenite is about 8%. Molybdenum in pure α-Fe the maximum solubility of about 32%, and decreased with temperature can precipitate Fe3Mo2, to obtain an age-hardening systems. Molybdenum is a ferrite forming element, capacity equivalent to chromium. In the martensitic stainless steel, molybdenum addition to improving the corrosion resistance, but also improve the hardness and strength, and enhance the secondary hardening effect of steel. The role of the stainless steel molds and cutting tools is very beneficial, usually in the martensitic stainless steel in the ω (Mo) <1%.

Case Hardening Stainless Steel Surface Use Kolsterising By yaang.com

The Kolsterising improves the wear resistance of stainless steel surface, without degrading their corrosion resistance. There are no additions of chemical elements to the steel during the process.

It is claimed that austenitic stainless steel which contain molybdenum, such as 1.4401 (316) can also have enhanced corrosion resistance after Kolsterising.The Kolsterising process does not apply a coating on the surface but is a low temperature surface carbon diffusion treatment. Although large quantities of carbon are diffused into the surface visiblechromium carbides are not formed.

Surface Hardening of Austenitic Stainless Steel with Nitrogen By yaang.com

Austenitic stainless steel cannot be through hardened, so whilst they remain a preferred choice of stainless steel for many applications they are very susceptible to wear and galling. One common treatment used to increase the surface hardness of such steel and to minimise galling is to Nitride the steel by Plasma or Salt Bath Nitriding. This provides a very hard (>1000Hv) surface however there is an associated loss of corrosion resistance in the nitride layer. 

Selection of Stainless Steel for Handling Phosphoric Acid H3PO4 By yaang.com

Phosphoric Acid is also know as orthophosphoric acid and is classed as a weak acid. Austenitic stainless steel have goodcorrosion resistance to chemically pure phosphoric acid. Wet process phosphoric acid (WPA) can be aggressive.

It is used as a chemical-cleaning agent for stainless steel but is not considered to be a 'passivating' acid.

Selection of Stainless Steel for Cryogenic Application By yaang.com

Ferritic, martensitic and duplex stainless steel tend to become brittle as the temperature is reduced, in a similar way to other ferritic / martensitic steel. The austenitic stainless steel such as 304 (1.4301) and 316 (1.4401) are however 'tough' at cryogenic temperatures and can be classed a 'cryogenic steels'. They can be considered suitable for sub-zero 'ambient' temperature sometime in service specification sub-arctic and arctic application and location (typically down to -40°C).

Chloride Stress Corrosion Cracking (Stainless Steel) By yaang.com

The most effective means for preventing stress corrosion cracking SCC are proper design, reducing stress, removing critical environmental contributors (for example, hydroxides, chlorides, and oxygen), and avoiding stagnant areas and crevices in heat exchanger where chlorides and hydroxides might become concentrated.  Low alloy steel are less susceptible than high alloy steel, but they are subject to SCC in water containing chloride ions.  Nickel based alloys are not affected by chloride or hydroxide ions.

Compared Ferritic Stainless Steel with Duplex Stainless Steel By yaang.com

High content of alloying elements, the price is relatively high, generally free of nickel ferrite. 

In summary, you can generally see the use of DSS performance and process performance of the general picture that, with its superior mechanical and corrosion-resistant comprehensive performance to win the favor of users, both to save weight, it has become an excellent resistance to save investment Corrosion Engineering.

The Benefits to Using Duplex Stainless Steel By yaang.com

The attractive combination of high strength, wide range of corrosion resistance, moderate weldability would seem to offer great potential for increasing the market share of duplex stainless steel. However, it is important to understand the limitations of duplex stainless steel and why they are always likely to be “niche players”. 
The advantage of high strength immediately becomes a disadvantage when considering formability and machinability. The high strength also comes with lower ductility than austenitic stainless steel grades. Therefore, any application requiring a high degree of formability, for example, a sink, is ruled out for duplex stainless steel grades. Even when the ductility is adequate, higher forces are required to form the material, for example in tube bending. There is one exception to the normal rule of poorer machinability, grade 1.4162.

Stress Corrosion Cracking SCC Of Duplex Stainless Steel By yaang.com

SCC Stress Corrosion Cracking Of Duplex Stainless Steel is a form of corrosion which occurs with a particular combination of factors:

• Tensile stress
• Corrosive environment
• Sufficiently high temperature. Normally 50 deg C but can occur at lower temperatures around 25 deg C in specific environments, notably swimming pools.

Corrosion Resistance of Duplex Stainless Steel By yaang.com

The range of duplex stainless steel allows them to be matched for corrosion resistance with the austenitic stainless steel and ferritic stainless steel grades. There is no single measure of corrosion resistance. However, it is convenient to use the Pitting Resistance Equivalent Number (PREN) as a means of ranking the grades. 
PREN = %Cr + 3.3 x %Mo + 16 x %N
The following table shows how the duplex stainless steel compare with some austenitic stainless steel and ferritic stainless steel grades.

Principle of Duplex Stainless Steel By yaang.com

The idea of duplex stainless steel dates back to the 1920s with the first cast being made at Avesta in Sweden in 1930. However, it is only in the last 30 years that duplex stainless steel have begun to “take off” in a significant way. This is mainly due to advances in steelmaking techniques particularly with respect to control of nitrogen content. 

The standard austenitic stainless steel like 304 (1.4301) and ferritic stainless steel like 430 are relatively easy to make and to fabricate. As their names imply, they consist mainly of one phase, austenite or ferrite. Although these types are fine for a wide range of applications, there are some important technical weaknesses in both types: 

Martensitic and Precipitation Austenitic Stainless Steel Comparison Table By yaang.com

Martensitic and Precipitation Austenitic Stainless Steel Comparison Table is intended to relate former BS, EN, German and Swedish grade designations to the current EN steel numbers, AISI grades and UNS numbers. The table is based on the 'wrought' ie long products (stainless steel bars etc), flanges products (stainless steel flanges etc) stainless steel numbers published in EN 10088 and related standards.
Casting products use different composition and so have their own stainless steel numbers in EN 10283. The related castings grades in both EN 10283 and BS 3100 are included in the table.

Effect of Cold Work and Heat Treatment of Austenitic Stainless Steel By yaang.com

Austenitic stainless steel are generally non magnetic with magnetic permeabilities of around 1.0. Permeabilities above 1.0 are associated with the amount of either ferritic or martensitic phases present in the austenitic stainless steel and so depend on:

• cold working and heat treatment conditions
• chemical composition effects

Cold Working and Heat Treatment

Composition Effect Magnetic Permeability of Austenitic Stainless Steel By yaang.com

Austenitic stainless steel are generally non-magnetic with magnetic permeabilities of around 1.0. Permeabilities above 1.0 are associated with the amount of either ferritic or martensitic phases present in the austenitic stainless steel and so depend on:

• chemical composition
• cold working and heat treatment conditions

Here discusses chemical composition effects.

Mechanism Measurement of Work Hardening Austenitic Stainless Steel By yaang.com

Austenitic stainless steel work-hardening significantly during cold working. This can be both useful properties, enabling extensive forming during stretch forming without risk of premature fractures and a disadvantage, especially during machining, requiring special attention to cutting feeds and speeds.

Stress Relieving Heat Treatment for Austenitic Stainless Steel By yaang.com

Unlike martensitic stainless steel, the austenitic stainless steel are not hardenable by heat treatment as no phase changes occur on heating or cooling. Softening is done by heating in the 1050/ 1120 °C range, ideally followed by rapid cooling. This is of course the complete opposite to martensitic steel, where this sort of treatment would harden the steel.

Apart from inter-stage annealing during complex or severe forming operations, for many applications, final stress relievingaustenitic stainless steel products is not normally needed.

Effect of residual stresses

Cold worked austenitic stainless steels will contain some 'strain induced' martensite, which, as well as making the steel partially 'ferro-magnetic', can also reduce the corrosion resistance. A highly stressed cold worked structure may also have lower general corrosion resistance than a fully softened austenitic structure.

The main hazard is stress corrosion cracking (SCC), which relies on tensile strength as part of the failure mechanism. Stress relieving removes such residual tensile stresses and so improves the SCC resistance.

The other main reason for stress relieving is to provide dimensional or shape stability. The risk of distortion can be reduced during forming or machining operations by stress relieving.

The approach to heat treatment selection

A full solution anneal stress-relieving heat treatment will re-transform any martensite formed back to austenite. (This will also give the lowest magnetic permeability possible for any particular grade.) Slow cooling is advisable to avoid introducing distortion problems or residual thermal tensile stresses and so the risk of sensitisation during a slow cool may have to be accepted.

The temperature ranges used in stress relieving must avoid sensitising the steel to corrosion or the formation of embrittling precipitates. As a general guideline, it is advisable that the range 480-900C is avoided. The low carbon (304L or 316L) or the stabilised (321 or 347) types should not be at risk from corrosion sensitisation during stress relieving treatments.

Stress relieving treatments for austenitic stainless steel

The table shows alternative treatments in order of preference.

Process or Corrosion Hazard	Stainless Steel Grade Types
	Standard Carbon 304, 316	Low Carbon 304L, 316L	Stabilised 321, 347
Annealing following severe forming	C	A,C	A,C
Forming interstage annealing	C(A,B)	A,B,C	B,A,C
Post welding heavy sections and/or high service loading applications	C	A,C,B	A,C,B
Dimensional stability	D	D	D
Severe SCC risk in service	Note 1	A,B	B,A
Some risk of SCC in service	C	A,B,C	B,A,C

Note 1
Standard carbon steel grades are susceptible to intergranular corrosion (ICC) on slow cooling treatments. Fast cooling treatments are not advisable as residual tensile stresses could result in SCC.

Note 2
Treatment B is also intended to reduce the risk of "knife-line" attack in the stabilised grades. This form of attack is due to the solution of titanium or niobium carbides at higher annealing temperatures.

Heat Treatment Codes

Code	Treatment Cycle
A	1050 / 1120 °C, slow cool
B	900 °C, slow cool
C	1050 / 1120 °C, fast cool
D	210 / 475 °C slow cool (approx. 4 hours per 25mm of section)

Source: Zhejiang Yaang Pipe Industry Co., Limited (www.yaang.com)

Compared Duplex Stainless Steel and Austenitic Stainless Steel Strength and Vulnerable By yaang.com

Compared with the austenitic stainless steel, duplex stainless steel of the vulnerable is as follows:

1 The application of universal and multi-faceted as stainless steel, such as its temperature must be controlled at 250 degrees Celsius.

2 The plasticity and toughness lower than the austenitic stainless steel, cold, heat processing process and molding properties than austenitic stainless steel.