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Corrosion Resistant Alloys
Description of material
SG1 is basically commercially pure Nickel with excellent corrosion resistance in both reducing and oxidizing media. Its ferromagnetic behavior offers a low coercive force and good magnetic permeability that, together with good electrical conductivity, allows to find applications in many industrial fields. SG1 is normally produced as Nickel 201 i.e. with a very low Carbon content.
Thanks to its magnetic properties, SG1 is widely use in the fabrication of several kinds of electronic, electric and magnetic components associated with high resistance to many kinds of corrosion. Therefore, SG1 is suitable to produce solenoid cores or electro- valves where the typical ferritic SS grade don’t offer adequate corrosion resistance. It is also used for fittings, tanks, valves, heat exchangers, parts for food industries and piping/hollows for manipulation/transport of caustic and alkali solutions.
SG1 is resistant in neutral, reducing and oxidizing environments. In this last situation, it offers a feeble or not so high resistance despite the formation of a passive film. Its composition warrants an outstanding resistance to both Chloride and caustic SCC. It has poor or low resistance in some kinds of oxidizing minerals acid at certain levels of concentration and in Sulphur atmospheres typical of fuel, oils and natural gas used in furnaces for heating in hot deforming processes. In fresh and drinkable water it provides good resistance. However in stagnant seawater resistance could be strongly decreased if its surface was covered by micro-organisms and fouling.
SG1 has a cold working hardening factor to similar, or lower, to comparable Copper austenitic grades such as AISRUH and can fabricated by cold working operations such as cold drawing and bending. It could even be used for a moderate amount of cold heading, provided that this cold processing operation is carried out in the annealed condition. High levels of cold working require applying an intermediate annealing. The temperature of annealing should be well evaluated in order to maintain a fine grain structure and mechanical properties. In short: lower temperatures give fine grain and high strength while higher temperatures could cause a recrystallization whose consequence is coarsening grains and lower strength.
SG1 has the typical poor machinability of a softer austenitic structure of grades that are Copper alloyed, such as AISRUH. Even though the productivity gain depends on types of machines used, the kind of tools used and their geometry, cutting fluids and the kind of machine operations on the pieces being produced, difficulties could happen in drilling, turning, threading and milling processes due to low chip-ability, and stringy chips which have a tendency to create a built-up edge. A correct choice of both cutting fluids and a right dimension of chip breakers helps to reduce the typical machining difficulties of this alloy. Moreover, an increasing of machinability and roughness of machined parts could be improved by a harder structure obtained by a cold drawing process and by dissipating heat using an appropriate and large amount of cutting fluids and tools with a correct edge geometry.
SG1 can be welded by using any one of welding process applied with typical austenitic grades but requires some different welding process evaluations when compared to these ones. Evaluations about the joint design should be considered, both in terms of lower weld penetration and poorer mobility of molten metal in the fused zone. Correct welding practices such as right heat inputs, inert shielding gas and cleanliness before/after welding must be followed to obtain best results. No preheating or post welding heat treatment are normally necessary but the weld discoloration should be removed. SG1 requires special filler metals to obtain a high corrosion resistance and toughness of the weld.
SG1 has a forging temperature lower than that of typical austenitic stainless steels. Therefore, a precise control of forging temperature is recommended. Overheating and reductions at temperature closest to low range of forging must always be avoided. . Therefore, the choice of hot working temperature and process parameters must evaluate both the strain rate and the consequent increasing of temperature that is reached after hot deformation. High strain rates and temperatures at the top of the range during the hot forming process, could generate structural loss of cohesion or internal bursts and large grain size. Obtaining a fine grain structure is very important for mechanical, fatigue and corrosion resistance properties and make it easier for ultrasonic testing to detect small indications as required by several International Norms. As with most Ni-Alloys, the furnace atmosphere shall be free of or have a low Sulphur content. Forgings should be air or forced air cooled.