The nickel based alloys are used in the wider range of applications as corrosion resistant alloys, resistance heating elements, controlled expansion alloys and elevated temperature creep resistant alloys. Days are gone when the emphasis was on the initial cost of the material. Only little concern was given to the maintenance and downtime related with the apparatus. In the modern time, increased maintenance cost and downtime have given greater emphasis on the consistent functionality of the process apparatus. The annual cost of the apparatus over the reasonable life is essential now in the material choice.

Nickel is more stable than iron but more reactive than copper. In the reducing conditions like sulfuric acid, nickel offers better corrosion resistance than iron however lower than copper or cupronickel alloys. The Nickel-molybdenum alloys are better corrosion resistant in reducing conditions than nickel or cupronickel alloys.

Nickel can produce a secured layer in some conditions, it is not a specifically stable layer, hence it cannot be normally employed in the oxidizing environments like nitric acid. The combination of nickel with chromium offers a greatly enhanced stable passive film results, providing greater corrosion resistance to the different oxidizing conditions. Although, these are subjected to corrosion in conditions containing chloride or other halides, particularly if oxidizing media is available, pitting attack occurs. The corrosion resistance can be enhanced by including molybdenum and tungsten.

The major feature of nickel in association with the production of corrosion resistant alloys is its metallurgical compatibility with several other metals like chromium, molybdenum, copper and iron. Generally nickel alloys are austenitic in nature although they can be subjected to the precipitation of intermetallic and carbide phases upon aging. In few alloys that are made for elevated temperature applications, intermetallic and carbide precipitation reactions are promoted to enhance properties. Although for corrosion applications, the precipitation of second phases often increases corrosion. The problem hardly occurs because alloys are introduced in the annealed condition and service temperatures hardly approach the level needed for sensitization.

The general corrosion is an essential factor, the major reason of using nickel based alloys in several industrial applications is their outstanding resistance to localized corrosion like pitting and crevice corrosion and stress corrosion cracking. In several conditions, austenitic stainless steels are resistant to general corrosion however they receive considerable localized corrosion resulting in large downtime and costly repair and replacement.

The localized corrosion resistance of alloys is enhanced by molybdenum but molybdenum solely cannot solve the problem. For instance, Hastelloy B-2 containing molybdenum in the maximum percentage is not suitable for use in the localized corrosion. On the other side, alloy B2 containing chromium is also significant for use in the oxidizing conditions.

The nickel based alloys are named as superalloys as they possess excellent high temperature strength and oxidation resistance.

Nickel 200 & Nickel 201: Nickel alloys 200 (UNS N02200) and 201 (UNS N02201) have physical and mechanical properties similar to copper. The curie temperature at which its magnetism changes with the kind and magnitude of alloy elements, increases with increasing in content of iron and cobalt and decreases when copper, silicon and other elements are included. Nickel is an essential alloying agent of various classes of corrosion resistant alloys.

Physical and mechanical properties of Nickel 200/201

Property	Nickel 200	Nickel 201
Modulus of elasticity	28 x 10 (6) psi	30 x 10 (6) psi
Tensile strength	27 x 10 (3) psi	58.5 x 10 (3) psi
Yield strength 0.2 % offset	21.5 x 10 (3) psi	15 x 10 (3) psi
Elongation in 2 inch	47 %	50 %
Brinell hardness	105 brinell	87 brinell
Density	0.321 lb per in3	0.321 lb per in3
Specific heat	0.109 Btu/lb oF	0.109 Btu/lb oF
Thermal conductivity
0 to 70 oF	500 Btu / hr ft2/ oF/ inch	569 Btu / hr ft2/ oF/ inch
70 to 200 oF	465 Btu / hr ft2/ oF/ inch	512 Btu / hr ft2/ oF/ inch
70 to 400 oF	425 Btu / hr ft2/ oF/ inch	460 Btu / hr ft2/ oF/ inch
70 to 600 oF	390 Btu / hr ft2/ oF/ inch	408 Btu / hr ft2/ oF/ inch
70 to 800 oF	390 Btu / hr ft2/ oF/ inch	392 Btu / hr ft2/ oF/ inch
70 to 1000 oF	405 Btu / hr ft2/ oF/ inch	410 Btu / hr ft2/ oF/ inch
70 to 1200 oF	420 Btu / hr ft2/ oF/ inch	428 Btu / hr ft2/ oF/ inch
Coefficient of thermal expansion
0 to 70 of	6.3 x 10(-6) (inch / inch/oF)	–
70 to 200 oF	7.4 x 10(-6) (inch / inch/oF)	7.3 x 10(-6) (inch / inch/oF)
70 to 400 oF	7.7 x 10(-6) (inch / inch/oF)	–
70 to 600 oF	8 x 10(-6) (inch / inch/oF)	–
70 to 800 oF	8.3 x 10(-6) (inch / inch/oF)	–
70 to 1000 oF	8.5 x 10(-6) (inch / inch/oF)	–
70 to 1200 oF	8.7 x 10(-6) (inch / inch/oF)	–

Maximum design stress for Nickel 200 and 201

Service temperature		Maximum stress
		4 inch dia. Pipe		5 to 12 inch dia, pipe
		Nickel 200	Nickel 201	Nickel 200	Nickel 201
100 of	38 oC	10000 psi	8000 psi	8000 psi	6700 psi
200 of	93 oC	10000 psi	7700 psi	8000 psi	6400 psi
300 of	149 oC	10000 psi	7500 psi	8000 psi	6300 psi
400 of	204 oC	10000 psi	7500 psi	8000 psi	6200 psi
500 of	260 oC	10000 psi	7500 psi	8000 psi	6200 psi
600 of	316 oC	10000 psi	7500 psi	8000 psi	6200 psi
700 of	371 oC	–	7400 psi	–	6200 psi
800 of	427 oC	–	7200 psi	–	5900 psi
900 of	482 oC	–	4500 psi	–	4500 psi
1000 of	538 oC	–	3000 psi	–	3000 psi
1100 of	593 oC	–	2000 psi	–	2000 psi
1200 of	649 oC	–	1200 psi	–	1200 psi

Nickel alloy 201 is a low carbon grade of alloy 200. Nickel 200 is subjected to the production of a grain boundary graphite phase that decreases ductility extensively. Resulting Nickel alloy 200 offers maximum temperature service up to 600oF or 315oC. However alloy 201 is utilized in the applications above this temperature limit.

The corrosion resistance of Nickel 200 and 201 are same. They attain excellent resistance to warm alkalies, specifically caustic soda. Outstanding resistance is offered to the whole concentrations at temperatures up to melting point. Lower than 50 percent, corrosion rates are nominal, often less than 0.2 mpy even in the boiling solutions. With increase in concentration and temperature, corrosion rates increase slightly. The contaminants in caustic like chlorates and hypochlorites will demonstrate the corrosion rate.

Nickel is prone to stress corrosion cracking in the chloride salts, and it attains outstanding resistance to nonoxidizing halides. The oxidizing acid chlorides like ferric, cupric and mercuric are very attacking and should be prevented.

Nickel alloy 201 has popular applications in handling warm, dry chlorine and hydrogen chloride gas on a regular basis about 1000oF or 540oC. The resistance is featured to the production of a nickel chloride layer. Dry fluorine and bromine are also handled. The corrosion resistance decreases in presence of humidity.

Nickel attains outstanding resistance to several organic acids, specifically fatty acids like stearic and oleic acids, in moderate aeration. Nickel is also resistant to anhydrous ammonia or ammonium hydroxide in contents about 1 percent or small. Larger alkali magnitudes can cause quick corrosion. Nickel is also used in applications of food and synthetic fibers due to their ability to sustain the product purity. The availability of nickel ions is not harmful to food flavors meanwhile it is also not poisonous.  Opposite to iron and copper, nickel doesn’t discolor organic compounds like phenol and viscous rayon. Besides of alloy 200, there are several alloy enhancements made to achieve large strength, toughness, galling resistance and increased corrosion resistance.

Welding & Brazing

The joining of nickel and nickel alloys can be consistently performed following the various welding methods. The ductile and solution strengthened nickel and its alloys are easily weldable and in particular these are suitable with all types of welding procedures. The weld forming of these alloys is easy and they often do not need heating before and after the process, and the interpass temperature control while welding is not much crucial. The precipitation hardenable alloys are less likely weldable and due to the availability of gamma prime reinforcing phase they become prone to strain age cracking. They are normally welded in annealed or solution annealed conditions and are postweld heat processed to precipitate gamma prime phase as an eventual or almost ultimate formation step. The precipitation hardenable alloys are also normally welded by brazing.

Welding features

Nickel is a conducting metal and copper is an excellent conducting metal. Although the inclusion of chromium or iron decreases the thermal conductivity of nickel or cupronickel. Both thermal and electrical conductivity decrease when solvent mixes with the solute atoms to produce an alloy. Thermal conductivity has a crucial role in the welding as it affects heat flow and magnitude of heat needed to melt material in the weld puddle.

The material chemistry with large thermal conductivity  conducts heat from fusion zone and hence needs nominally larger heat supply while welding than for alloy with small thermal conductivity that keeps the local heat of welding for a prolong period.

The nickel based alloys have variable thermal conductivity properties. The Ni-Cr and Ni-Cr-Fe alloys have values smaller than those of carbon steels and austenitic stainless steels whereas nickel and nickel-copper alloys have significantly larger values than steels.

The electric resistivity of a material is particularly essential in welding procedure since the melting rate of an electrode or wire is emphasized by electrical properties of the material. Often solid solution alloys are not excellent electric conductors, as the solute and solvent atoms are of different sizes. The condition deforms the crystal lattice structure of the material and obstructs the flow of current. The alloys that are constituted basically of single element like nickel 200 and nickel 201 have minimum electric resistance.

The thermal expansion properties of a material are essential as the heat that is supplied to the weld joints results in the localized expansion or contraction while heating or quenching. The material with larger thermal expansion coefficient expands or contracts significantly while the welding operation hence developing a large stress on the weld joint. It is specifically noticeable when the weld deposit is susceptible to cracking. It should also be taken into account while welding of unlike metal particularly when there is a great variation in their expansion properties.

Liquidus – Solidus Temperature: The cracking resistance of a material is usually linked with the variation in liquidus and solidus temperatures, specifically when stress is included. When the restrain alloy shape is welded, large stress is produced while the welding procedure. Having a big difference between liquidus and solidus temperatures, keeps a material in the cracking conditions for a significant period. Although the limited difference offers less time for generation of cracking.

Welding metallurgy of heat influenced region

The heat affected region is stated as the area of the base metal that doesn’t melt but its mechanical characteristics alter due to heat produced by welding.

Grain boundary precipitation: Grain boundaries are a weak connection in a microstructure that is subjected to the high temperatures whether in service or in welding, while the thermal cycles that are a part of welding procedure. The precipitates that produce in the grain edges may change in size and chemistry and can be detrimental or advantageous. The intergranular carbides in Inconel 600 and alloy 690 can prevent stress corrosion cracking in the high purity water conditions in nuclear reactor steam generator, basically side conditions.

Grain Development: It is more straightforward to create flawless welds in the materials of fine grain size, as there is an unwanted element content that has a role in grain boundary fracture in a type of fine grain network. With wider grains, there are some grain boundaries and the damaging elements present in high content in them. Although it is not always feasible to create a fine grain configuration for welding. Several elevated temperature applications require a big grain size material to confirm high temperature creep and rupture strength.

While welding of a fine grain material, grain growth is noticed in the heat affected region. The grain growth band’s width is based on the several factors – type of welding process, heat input, flow speed and heat distribution properties of the base metal. The electron beam welding procedure creates a thinner strip of heat affected regions and a smaller band of grain development as compare to that in gas metal arc welding procedure.

Hot Fracture: Considering the constitutional liquation, heat affected zone cracks are rarely an issue in the nickel or cupronickel alloys however they have been considered for Ni-Cr and NiCrFe alloys.

These cracks have been noticed in Inconel 600 when the large heat supply about 2.4 MJ per meter or above is utilized. These cracks are normally tight, linear intergranualr separations vertical to the weld flow direction. The cracks have been noticed in the heat affected regions of welds created by gas metal arc welding, gas tungsten arc welding and electron beam welding methods.

Grain boundary precipitation is a common method that results in cracks. The liquation of boundary occurs upon melting of a grain boundary by a phase at temperature lower than melting point. Since the liquated grain limit has actually no strength, partition at the liquated grain boundary may cause cracking.

In the constitutional liquation, a stable second phase particle starts to dissolve while nonequilibrium or quick heating and creates a moderate phase with the matrix that has solidus temperature lower than second phase particle or matrix. In Inconel 600, some tests have recognized the precipitation of Cr7C3 at the grain boundary with the development of second phase particle.

The chemical separation at the grain limit causes grain boundary liquation. Boron and sulfur produce low melting temperature eutectics that can add to HAX fissuring. Manganese separation in alloy 600 results in the production of binary compound (manganese -38 Nickel) that has solidus temperature about 1000oC or 1830oF that is lower than its bulk melting point.

Various factors that reduce the development of heat affected zone cracking and control are:

1. Heat supply while welding
2. Microstructure to receive reasonably small grain size
3. Composition to decrease the magnitude of damaging minor elements that increase crack forming rate

Various elements that can increase heat affected zone cracking in nickel based alloys are purposefully added to improve the characteristics. However these agents are nominal because they are added in the mild percentage, they are far from being nominal in their influence on cracking. These elements are sulfur (S), phosphorous (P), lead (Pb), boron (B), zirconium (Zr), tin (Sn), zinc (Zn), selenium (Se), tellurium (Te), antimony (Sb), bismuth (Bi), gold (Au) and silver (Ag). The elements that are purposefully added are boron and zirconium, improve hot workability and high temperature strength of nickel base alloys but they are also of damaging nature in the welding procedures.

Welding metallurgy of fusion region

The fusion region is the base metal melted area while the welding process as observed on the weld cross-section. The fusion region of autogenous welds is described as sensitive to porosity production, hot fracturing susceptibility and microsegregation.

Considering the porosity, the nickel base alloys are categorized into classes – alloys with chromium and without chromium. Chromium element consists of a natural affinity for gases that produce while welding process significantly such as oxygen, nitrogen and hydrogen. As the nickel and cupronickel alloys have deficiency of chromium, these are susceptible to porosity production while autogenous welding. Hence, the security of the weld puddle while melting and solidification is crucial to receive porosity free autogenous welds in these two groups. Dry torch gases and sufficient gas security are needed. However using the filler metals contribute in controlling porosity by the inclusions of gas absorbing elements like titanium and aluminum, suitable care is still adequate.

Controlling porosity is extremely easy in the nickel- chromium alloys and nickel-chromium-iron alloys due to their large chromium magnitudes. It doesn’t refer to state that gas coverage is not essential in autogenous welding of alloys containing chromium but it is not important as compare to in nickel and cupronickel alloys.

Hot fracturing sensitivity

Hot fracturing or cracking is a grain edge based mechanism that occurs basically in the Ni-Cr and NiCrFe alloys. However the nickel and cupronickel alloys are not resistant to this experience, these occur more in the chromium containing nickel superalloys.

In welding process of Incoloy 800, it is observed that lowering aluminum and titanium percentage to very low quantity basically prevents hot cracking. It is also observed that in the presence of low magnitudes of these elements, separation of titanium and aluminum cause grain boundary embritllement and banding that is adequately intense to attain liquation while heating in a welding process. It also seems to have a powerful link among sulfur and titanium in the different additions noticed in the sulfur based alloys.

 Microsegregation: Upon solidification, a non-uniform dendritie structure forms. Solidification starts with the maximum melting point liquid and continues as long as the complete structure is solid. The microsegregation or coring takes place while this solidification occurs and regions of solidified microstructure may have a large variation in chemistry. It results in a problem in the corrosion conditions for which an alloy was selected for its specific corrosion resistance. For instance, as the dendritic weld material can possess extensively different chromium and molybdenum contents, specific corrosion may occur and travel at the regions of microstructure of element responsible for offering the corrosion resistance.

Microsegregation cannot be discarded in a weld, however it can be reduced by sensible choice of welding factors like controlled heat supply. Elevated temperature heat processing for long period can moderately balance the chemistry gradients through diffusion. Cold treating the weld structure, after annealing, can crack this microsegregation, if this kind of treatment is followed.