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Stainless steels are classified according to their matrix structure.

(a) austenitic (b) ferritic (c) martensitic

(d) precipitation hardened and (e) duplex.

Special features of stainless steels related to welding.

1. Low thermal conductivity (50% of mild steel) results in less heat input for the job and 10% less current is needed for SS electrodes. higher electrode melt. off rates are also obtained. Melting point of stainless steel is 93°C lower.

2. Thermal expansion of Cr-Ni steels is about 50% greater than for mild steel.

This increases the chances for warping and buckling. Thus suitable fixture must be used for welding stainless steels.

3. Electrical resistance is 6–12 times higher which causes overheating in the elec-trodes. Shorter electrodes are, therefore used to reduce electrode heating.

Austenitic stainless steels

1. These steels contain 16–26% chromium 6–22% Nickel.

2. Type 304 L and 316 L are low carbon grade (C ≤ 0.03%).

3. Mo in type 316 improves corrosion resistance and high temperature properties.

4. Types 321 and 347 stainless steels are stabilized against carbide (Cr₂₃C₆) precipita-tion, weld decay and intergranular corrosion by addition of Ti and Nb. The strong carbide formers form TiC and NbC which impart creep resistance. Hence they are also used as creep resisting steels.

5. The 200 series s.s. sin lower Ni which is compensated by Mn and N₂ for austenite formation.

6. Austenitic S.S. (except free machining grades) are easiest to weld and produced welds that are tough.

7. S.S. welding requires 20–30% less heat input than welds in carbon steels, because of low thermal conductivity and high electric resistance. Excess heat will cause distortion, reduce strength and corrosion resistance. Sulpher and Selenium added for free machining, makes the steel unweldable, also high carbon content inhibit weld serviceability. External sources of contamination include carbon nitrogen, oxygen, iron and water.

8. Contaminations and their effects.

• Carbon contamination may cause welds to cracks, change mechanical properties and reduce corrosion resistance in weld areas.

• Iron contamination lowers serviceability, flakes of iron on surface will rust, thus speed-ing localised corrosion.

• Contamination by copper, lead and zinc can lead to cracking in HAZ of the weld.

9. Welding current required is comparatively low.

10. When stainless steels are heated in the range of 427–870 C or cooled slowly through that range, carbon precipitates at grain boundaries.

11. Formation of these carbides effectively eliminates much of the chromium.

12. It will reduce corrosion resistance especially in HAZ.

13. This carbon precipitation can be minimized by :

(i) Reducing the time for which the temperature is between 427°–870°C range.

(ii) Selecting low carbon stainless steels to reduce carbide formation.

(iii) Addition of Ti, Ta, Columbium which form stable carbide preventing the formation of chromium carbide.

Carbide precipitation

1. Austenitic grades are non-hardening type and welding usually does not adversely affect weld strength and ductility. There is one detrimental effect of heating of Ni-Cr steel i.e., carbide precipitation at the grain boundaries resulting in reduced corrosion resistance. A fine film of Cr-rich carbides containing upto 90% Cr taken from metal layer next to grain boundary gets precipitated along the grain boundary. Precipitation of intergranular chromium carbides is accelerated by an increase in temperature within the sensitized range and by an increase in time at that temperature.

2. Carbide precipitation can be controlled by :

• Using stabilised steels, by adding columbium and titanium which have greater affinity for carbon than does chromium. Columbium is exclusively used for the purpose in welding electrodes as titanium gets lost in transferring across the arc.

• Rapid quenching may minimise carbide precipitation, but this may not always be possible specially in thick sections.

• Limiting carbon content to a maximum of 0.03% avoids carbide precipitation

• Post-weld solution annealing.

3. Solution annealing puts carbides back into solution restores corrosion resistance.

Austenitic S.S. with stabilization using Nb + Ti or Tantalum and welded with stabilised filler metal gives good strength and corrosion resistance properties.

4. SMAW process is widely used. A large number of electrodes available make the process widely acceptable. Some examples are given below:

• E308-16 electrode–metal transfer is spray type–smooth bead (AC or DCRP)

• Lime covered basic electrodes (only DCRP)–E308-15-globular transfer rough bead

• For heavy flat pieces SAW is used

• For thin sections TIG is excellent

• For sheets spot welding can be used.

Cracking

Interdendritic cracking in the weld area that occurs before the weld cools to room tem-perature is known as hot cracking or microfissuring. Weld metal with 100% austenite is more susceptible to microfissuring than weld metals with duplex structure of delta ferrite in austenite.

Susceptibility can be reduced by a small increase in carbon or nitrogen content or by a sub-stantial increase in manganese content.

To avoid solidification, cracking, weld metal should have a ferrite content of at least 3-5 ferrite number (FN) and hence filler metal of suitable composition is to be selected. For this purpose Schaeffler diagram is made use of; A modified version of it is h shown in Fig. 7.3 which takes care of nitrogen in the metal.

Nitrogen strengthened austenitic stainless steels offer the advantages of:

• Increased strength at all temperatures (cryogenic to elevated)

• Improved resistance to pitting corrsion

Austenite

Austenite

Chromium equivalent = % Cr+%Mo+1.5×%Si+0.5×%Nb

21 22 23 24 25 26 27

Fig. 7.4 De Long diagram They differ from conventional austenitic steels in that

• Mn substitutes a part of Ni, this allows more nitrogen to get dissolved in matrix of the alloy.

• Nitrogen acts as solid solution strengthener with increased annealed strength to approximately twice that of conventional austenitic steels.

Control of nitrogen content is important.

• Very low nitrogen lowers strength and corrosion resistance.

• Very high nitrogen causes porosity and hot cracking.