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.

superplastic forming

After developing welding and forming commercially pure titanium and titanium alloy Ti-6Al-4V, we turned our attention to superpastic forming of Ti-6Al-4V.We had been manufacturing gas bottles of titanium alloy for missile application. We wanted to examine whether we can adopt superplastic forming for manufacture of gas bottles.

Superplasticity is the property of certain metals and alloys to be stretched many times their intial length without fracture.Elongations of the order of 800% to 1000% can be expected if certain conditions such as temperature, strain rate, microstructure of the alloy can be controlled.Ti-6Al-4V shows excellent superplastic forming properties in the temperature range of 750degC to 1000degC.Maximum elongation is observe around 930 deg C. The Microstructure must be fine grained and the deforming temperature must be higher than half th melting temperature.In addition to the grain size, grain size distribution. grain aspect ratio and percentage of alpha -beta in the microsructure are also important. Without getting in to more details on theory of superplastic forming let us look into how we did free blowing of a sphere and some other shapes.

We selected a 3mm thin Ti-6Al-4V sheet for free blowing. We attempted free blowing a sphere. Circular blanks were trepanned by holding the plate in a vacuum chuck. The two blanks were held in a jig and the edges were welded by TIG welding. In of the blanks a thick walled tube of commercially pure titanium was welded. The welded blank was coated with a protective coating which reduced oxidation of the blanks during heating, An electric furnace which was earlier calibrated with the temperature uniformity of plus or minus 5degC was used for experiments. A thermocouple was attached to the nozzle and the temperature indicated by digital temperature recorder was accepted as the temperature of blank. The furnace was purged with argon and low volume flow was maintained throughout blowing,When the temperature of the blank reached 930 degC Argon was admitted in to the sandwich and the pressure was increased in increments of few Psi.After the assuring that the free blowing was completed, the pressure was released slowly and the furnace switched off. After the sphere was cooled to room temperature the surface was examined and the thickness was measured by ultrasonic thickness gauge.The thickness at the pole of the sphere was between 2.1mm to 2,2 and at the pole was 1,7mm to 1.8mm.

Although we demonstrated the feasibility of forming a sphere by free blowing we realised that the process is slow and hence we abandoned further trails. We continued with hot forming, machining,welding and testing. But if the number of components required was not many free blowing is a viable option.

Welding Maraging Steel

Maraging steel is vacuum induction melted plus vacuum arc melted, low carbon Nickel-Cobalt-Molybdenum High Temperature Alloy capable of attaining yield strength in excess of 240 ksi through a heat treatment. The steel exhibits good ductility coupled with high strength and is readily weldable. A aging heat treatment in the temperature range of 454-514 deg C from the solution treated condition will results in high strength.

The chemical composition of the steel is given below:

C-0.03max Mn-0.10max P-0.01max S-0.01max Si-0.10max Ni-18.50 Mo-4.80
Co-7.50 Ti-0.40 Al-0.10 B-0.003 Zr-0.01

Welding was performed on two thicknesses,namely, 5mm and 10mm. Gas tungsten arc welding was adopted using ultra high purity Argon. Matching filler wire was used in all welding trials.Test coupons were welded in solution annealed condition and the weldment was aged after welding and also welding was carried out after solution treatment and aging. The aim was to optimise heat-input to ensure that the development meets the stringent fracture toughness specified apart from meeting the tensile requirements. All the experiments done and the results obtained were compiled in a report which was handed over to MIDHANI which in turn forwarded the report to the customer. This was one of the contract development by R&D of BHPV.