Hastelloy-G welding

In late 1970's the emission from existing power plants and chemical plants came under Governments scrutiny and laws were enacted to control emission of sulphur di oxide phosphoric acids and other harmful emissions. Flue gas desulpharisation was insisted upon and there was a need to look in to the material which can withstand corrosive products emitted by the stack. We looked in to the problem from material angle and decided that we need to study Hastelloy-G as a candidate material.

Hastelloy-G is a columbium stabilised nickel based alloy with additions of chromium,molybdenum and iron as major constituents in the alloy. The chemical composition of this alloy
is chosen to resist both acidic and alkaline media. It can also resist both oxdising and reducing media. This alloy is eminently suited to handle sulphuric acid which changes from oxidising to reducing depending upon concentration and temperature. Hastelloy-G has outstanding resistance to mixed acids,flurosilisic acid, sulphate compounds,contaminated nitric acid,flue gases and hydrofluoric acid. The alloy resists pitting, crevice corrosion and also stress corrosion cracking.

Hastelloy-G is welded either by manual metal arc or by TIG process.In manual metal arc welding electrode confirming to ENiCrMo-1 which deposits a weld metal whose composition is close to plate material is used.Some minor problems are encountered during welding. Hastelloy-G being a nickel rich material flowability is poor which makes it difficult to place the weld where it is required. But this problem can be over come by training of welders.

Hastelloy-G plates are supplied in solution treated condition. Solution treatment consists of heating to 1175 degC, holding at this temperature for sufficient length of time and rapid air cooling or water quench.

Hastelloy- G being high nickel alloy the flowability of molten metal is poor. It is necessary to place the weld metal where it is required. Large bevel angles and small root face is necessary. Side wall fusion is another problem with welding of Hastelloy-G- It is difficult to obtain full penetration. In such cases back chipping and placing sealing weld is necessary to ensure good radiography quality welds. Magnetic arc blow which is problem with all high nickel alloy is also experienced by Hastelloy-G. But the problem can be overcome by suitably locating ground clamps. Porosity is another problem which is encountered often. By using short arc and proper cleaning of base metal and baking the electrode porosity can be eliminated. A large number of trials were carried out by varying heat in put over a wide range. Mechanical tests were carried out to establish the strength and ductility of welds. From these results we established the optimum heat input for welding Hastelloy-G.

In TIG welding the filler wire used is ERNiCrMo-1. Similar problems which we experienced with manual welding were also experienced with TIG welding.

The test specimens from both the methods of welding were subjected to corrosion tests in sulphuric acid with concentration ranging from 10% to 70%. At 70% concentration the acid becomes oxidizing and the corrosion rate is extremely high.

Weldments showed excellent corrosion resistance in entire range of concentration in phosphoric acid.

Weldments were subjected to stress corrosion cracking tests in 42% boiling Magnesium Chloride solution. Hastelloy-G did not show any signs of cracking even after hundreds of hours of test.

We passed on these results to the manufacturing section of BHPV for fabricating equipment with Hastelloy-G.