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Steel Types

Types of Steel

A. FERRITIC
B. AUSTENITIC
C. MARTENSITIC
D. DUPLEX (LEAN & SUPER)
E. PRECIPITATION HARDENING(PH)
CARBON & ALLOY STEELS


A. FERRITIC

ADDITIONAL ELEMENTS

Chromium + >0.10% Cerbon  

GENERAL USE

Usually limited in use to relatively thin sections due to lack of toughness in welds. However, where welding is not required they offer a wide range of applications. Often chosen for their resistance to stress corrosion cracking.  

WELDABILITY

Cannot be hardened by heat treatment and not as formable as austenitic stainless steels. As these alloys can be considered to be predominantly single phase and non-hardenable, they can be readily fusion welded. However, a coarse grained HAZ will have poor toughness.  

AVOIDING WELDING IMPERFECTIONS The main problem when welding this type of stainless steel is poor HAZ toughness. Excessive grain coarsening can lead to cracking in highly restrained joints and thick section material. When welding thin section material, (less than 6mm) no special precautions are necessary. In thicker material, it is necessary to employ a low heat input to minimise the width of the grain coarsened zone and an austenitic filler to produce a tougher weld metal. Although preheating will not reduce the grain size, it will reduce the HAZ cooling rate, maintain the weld metal above the ductile-brittle transition temperature and may reduce residual stresses. Preheat temperature should be within the range 50-250 deg.C depending on material composition.

MAGNETIC

Yes

B. AUSTENITIC

ADDITIONAL ELEMENTS

Nickel, Manganese and Nitrogen

GENERAL USE These steels are the most common.  

WELDABILITY

A commonly used alloy for welded fabrications is Type 304 which contains approximately 18%Cr and 10%Ni. These alloys can be readily welded using any of the arc welding processes (TIG, MIG, MMA and SA). As they are non-hardenable on cooling, they exhibit good toughness and there is no need for pre- or post-weld heat treatment.  

AVOIDING WELDING IMPERFECTIONS Although austenitic stainless steel is readily welded, weld metal and HAZ cracking can occur. Weld metal solidification cracking is more likely in fully austenitic structures which are more crack sensitive than those containing a small amount of ferrite. The beneficial effect of ferrite has been attributed largely to its capacity to dissolve harmful impurities which would otherwise form low melting point segregates and interdendritic cracks. As the presence of 5-10% ferrite in the microstructure is extremely beneficial, the choice of filler material composition is crucial in suppressing the risk of cracking. An indication of the ferrite-austenite balance for different compositions is provided by the Schaeffler diagram. For example, when welding Type 304 stainless steel, a Type 308 filler material which has a slightly different alloy content, is used.

MAGNETIC

Usually not, but may depend on composition or work hardening

C. MARTENSITIC

ADDITIONAL ELEMENTS

Chromium + Carbon up to 1%  

GENERAL USE

Used where high strength and moderate corrosion resistance is required.  

WELDABILITY

Hardened and tempered much like carbon and low-alloy steels. The principal difference compared with welding the austenitic and ferritic grades of stainless steel is the potentially hard HAZ martensitic structure and the matching composition weld metal. The material can be successfully welded providing precautions are taken to avoid cracking in the HAZ, especially in thick section components and highly restrained joints.

AVOIDING WELDING IMPERFECTIONS

High hardness in the HAZ makes this type of stainless steel very prone to hydrogen cracking. The risk of cracking generally increases with the carbon content. Precautions which must be taken to minimise the risk, include: Using low hydrogen process (TIG or MIG) and ensure the flux or flux coated consumable are dried (MMA and SAW) according to the manufacturer’s instructions. Preheating to around 200 to 300 deg.C. Actual temperature will depend on welding procedure, chemical composition (especially Cr and C content), section thickness and the amount of hydrogen entering the weld metal. Maintaining the recommended minimum interpass temperature. Carrying out post-weld heat treatment, e.g. at 650-750 deg.C. The time and temperature will be determined by chemical composition. Thin section, low carbon material, typically less than 3mm, can often be welded without preheat, providing that a low hydrogen process is used, the joints have low restraint and attention is paid to cleaning the joint area. Thicker section and higher carbon (> 0.1%) material will probably need preheat and post-weld heat treatment. The post-weld heat treatment should be carried out immediately after welding not only to temper (toughen) the structure but also to enable the hydrogen to diffuse away from the weld metal and HAZ.  

MAGNETIC

Yes

D. DUPLEX (LEAN & SUPER)

ADDITIONAL ELEMENTS

Approx 50% ferritic and 50% austenitic

GENERAL USE

Higher strength than either ferritic or austenitic steels.

WELDABILITY

They are weldable but need care in selection of welding consumables and heat input. They have moderate formability but are resistant to stress corrosion cracking.

AVOIDING WELDING IMPERFECTIONS

Although most welding processes can be used, low heat input welding procedures are usually avoided. Preheat is not normally required and the maximum interpass temperature must be controlled. Choice of filler is important as it is designed to produce a weld metal structure with a ferrite-austenite balance to match the parent metal. To compensate for nitrogen loss, the filler may be overalloyed with nitrogen or the shielding gas itself may contain a small amount of nitrogen. 

MAGNETIC

Yes

E. PRECIPITATION HARDENING(PH)

ADDITIONAL ELEMENTS

Copper, Niobium and Aluminium

GENERAL USE

high chromium and nickel content, with the most common types having characteristics close to those of martensitic (plain chromium stainless class with exceptional strength) steels. Applications for PH stainless steels include shafts for pumps and valves as well as aircraft parts.

WELDABILITY

Can be machined to quite intricate shapes requiring good tolerances before the final aging treatment as there is minimal distortion from the final treatment. Corrosion resistant similar to standard Austenitic steel These alloys may susceptible to post-weld heat treatment cracking.

AVOIDING WELDING IMPERFECTIONS

A distinguishing feature of precipitation hardened alloys is that mechanical properties are developed by heat treatment (solution treatment plus ageing) to produce a fine distribution of particles in a nickel-rich matrix. Nickel and its alloys are readily welded but it is essential that the surface is cleaned immediately before welding. The normal method of cleaning is to degrease the surface, remove all surface oxide by machining, grinding or scratch brushing and finally degrease. Common imperfections found on welding are: Porosity Oxide inclusions and lack of inter-run fusion Weld metal solidification cracking Microfissuring Additionally, precautions should be taken against post-welding imperfections such as: Post-weld heat treatment cracking Stress corrosion cracking Porosity Porosity can be caused by oxygen and nitrogen from air entrainment and surface oxide or by hydrogen from surface contamination. Careful cleaning of component surfaces and using a filler material containing deoxidants (aluminium and titanium) will reduce the risk. When using argon in TIG and MIG welding, attention must be paid to shielding efficiency of the weld pool including the use of a gas backing system. In TIG welding, argon-hydrogen gas mixtures tend to produce cleaner welds. Oxide inclusions and lack of inter-run fusion: As the oxide on the surface of nickel alloys has a much higher melting temperature than the base metal, it may remain solid during welding. Oxide trapped in the weld pool will form inclusions. In multi-run welds, oxide or slag on the surface of the weld bead will not be consumed in the subsequent run and may cause lack of fusion imperfections. Before welding, surface oxide, particularly if it has been formed at a high temperature, must be removed by machining or abrasive grinding; it is not sufficient to wire brush the surface as this serve only to polish the oxide. During multipass welding, surface oxide and slag must be removed between runs. Weld metal solidification cracking Factors which control solidification cracking include alloy, welding process and welding conditions. For example, solidification cracking is a factor which limits the application of submerged arc welding, both with respect to applicable alloys and welding conditions. More generally, this type of cracking leads to restriction of weld shape, welding speed and technique. Microfissuring Similar to austenitic stainless steel, nickel alloys are susceptible to formation of liquation cracks in reheated weld metal regions or parent metal HAZ. This type of cracking is controlled by factors outside the control of the welder such as grain size or impurity content. Some alloys are more sensitive than others. For example, some cast superalloys are difficult to weld without inducing liquation cracks. Post-weld heat treatment cracking This is also known as strain-age or reheat cracking. It is likely to occur during post-weld ageing of precipitation hardening alloys but can be minimised by pre-weld heat treatment. Solution annealing is commonly used but overageing gives the most resistant condition. Alloy 718 alloy was specifically developed to be resistant to this type of cracking. Stress corrosion cracking Welding does not normally make most nickel alloys susceptible to weld metal or HAZ corrosion. However, when the material will be in contact with caustic soda, fluosilicates or HF acid, stress corrosion cracking is possible. After welding, the component or weld area must be given a stress-relieving heat treatment to prevent stress corrosion cracking.

MAGNETIC

Yes

CARBON & ALLOY STEELS

Carbon Steel
Plain carbon steels are ferrous alloys based on iron, carbon and small levels of other alloying elements such as manganese or aluminum. Carbon steels include soft, non-hardenable low carbon or mild steels such as 1020 as well as hardenable high carbon steels such as 1095. Steel alloys are used in a wide variety of applications in almost every industrial segment. Mild steels and low carbon steels can be fabricated easily by machining, forming, casting and welding. High strength low alloy (HSLA) steels are carbon steel with small amount of additional alloying elements.

Alloy Steel
Alloy steels are ferrous alloys based on iron, carbon and high to low levels of alloying elements such as chromium, molybdenum, vanadium and nickel. Alloy steels include hardenable high alloy steels, maraging steels and other specialty steel alloys. Steel alloys are used in a wide variety of applications in almost every industrial segment. Low alloy steels can be fabricated easily by machining, forming, casting and welding.

Low Alloy / HSLA
High strength low alloy (HSLA) steels are carbon steels with small amounts of alloying additions. The strengthening from the alloy additions allow thinner sections of material compared what would be required with a plain carbon steel. In many automotive and transportation applications, the use of HSLA steel results in lower overall vehicle weight.

Clad / Bimetal
The metal or alloy stock is a clad or bimetal material, which consists of two or more different alloys bonded integrally together.