Austenitic consumables

When selecting austenitic stainless steel or nickel alloy fillers, it is necessary to ensure that dilution from the base steel can be satisfactorily accommodated. The normal choice of austenitic consumables for MMA welding is from the types 23Cr 12Ni, 29Cr 9Ni or 20Cr 9Ni 3Mo, e.g. from BS 2926 1984 Grades 23.12, 29.9, or... 

Eutectoid structures are like eutectic structures, but much finer in scale. The original solid decomposes into two others, both with compositions which differ from the original, and in the form (usually) of fine, parallel plates. To allow this, atoms of B must diffuse away from the A-rich plates and A atoms must diffuse in the opposite direction, as shown in Fig. A1.40. Taking the eutectoid decomposition of iron as an example, carbon must diffuse to the carbon-rich FejC plates, and away from the (carbon-poor) a-plates, just ahead of the interface. The colony of plates then grows to the right, consuming the austenite (y). The eutectoid structure in iron has a special name it is called pearlite (because it has a pearly look). The micrograph (Fig. A1.41) shows pearlite. 

The main consumer of nickel is austenitic stainless sleel, which contains from 3,5 to 22% nickel and 16 to 26% chromium. In these steels, nickel stabilizes the austenite and enhances the ductility of the steel. Nickel, along with chromium, contributes to corrosion resistance. Up to amounts of about 9%, nickel adds strength, hardness, and toughness to many alloy steels. 

Nitric acid is normally stored in flat-bottomed, roofed tanks that are made from low-carbon, austenitic stainless steel. Most concentrations of nitric acid are available in tank cars and by truck. Stainless steel is necessary for concentrations up to 80% to 85%. Stronger solutions are less corrosive and may be stored in aluminum. Approximately 90% of nitric acid production is consumed on site to make ammonium nitrate (AN) fertilizers and some industrial explosives. Thus, the merchant market for nitric acid accounts for only 10% of the total. The production of AN fertilizers and most industrial explosives do not require acid concentrations in excess of the azeotropic composition of 68.8%. ... 

When austenite grew into and consumed dendrites which formed initially as ferrite, thin regions of ferrite remained in the austenite matrix in some stainless steels, (for example in steel 403, figure 7). This solid phase transformation also gives rise to partitioning of alloying elements between austenite and dendritic ferrite, and a partition ratio Pd may be calculated ... 

The technique can be adapted to the analysis of the hydrogen contents of austenitic and nickel alloy consumables used to weld ferritic steels. However, only total hydrogen values are meaningful for these materials, as diffusion rates at ambient temperature are so slow (see Fig. 5.17) that a diffusible hydrogen value would be meaningless. There will be a need to use a higher collection temperature than with ferritic steels. 

The delta ferrite content of each lot and/or heat of weld filler metal used for welding of austenitic stainless steel code components shall be determined for each process to be used in production. Delta ferrite determinations for consumable inserts, electrodes, rod or wire filler metal used with the gas tungsten arc welding process, and deposits made with the plasma arc welding process may be determined by either of the alternative methods of magnetic measurement or chemical analysis described in Section III of the ASME Code. 

The greatest field of application for nickel is austenitic stainless steels. This steel type and the influence of nickel are dealt with in Chapter 24 Chromium. Stainless-steel production accounted for more than 60% of the world nickel demand in the year 2000. The chemical industry is a very big consumer of nickel for a multitude of applications, mainly in stainless steels. In our everyday life we also meet nickel-containing, austenitic stainless steels in kitchen sinks, laundry equipment and cutlery, in railway cars and tank trailers. For the oil rigs offshore, with their exceptional corrosion problems, special stainless steels with high molybdenum contents, have been developed. 

Once duplex base metal and welding consumables have been selected, it is then necessary to select joint designs and weld parameters that will produce welding heat inputs and cooling rates so as to produce a favorable balance of austenite and ferrite in the weld and HAZ. 

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