Nickel Welding Automation Using Robots

Nick­el and Cobalt Alloys:

Nick­el and Cobalt base alloys are two heat-resist­ing weld­ing mate­ri­als that are often grouped togeth­er because they are used for sim­i­lar pur­pos­es. From heat resis­tance to cor­ro­sion resis­tance, Nick­el and Cobalt base alloys, also known as the super-alloys, are the most impor­tant of the heat-resist­ing class.

Heat-resist­ing alloys are met­als devel­oped to with­stand the severe con­di­tions pre­vail­ing in ser­vice at ele­vat­ed tem­per­a­tures where oth­er more com­mon mate­ri­als would fail.

What char­ac­ter­is­tics are required even when not weld­ing heat-resist­ing materials?

• Resis­tance to oxi­da­tion and scal­ing, ade­quate mechan­i­cal prop­er­ties and resis­tance at high tem­per­a­ture, both for short time (hot ten­sile strength) and extend­ed time (creep resistance).
• Sta­bil­i­ty with time. The severe ser­vice con­di­tions affect­ing these mate­ri­als may induce changes in struc­ture and prop­er­ties, with fur­ther for­ma­tion of cracks.
• Duc­til­i­ty and resis­tance to high tem­per­a­ture inter­gran­u­lar attack (IGA) are also impor­tant. Weld­ing heat-resist­ing alloys should not degrade these properties.

Iron Base Alloys

Heat resist­ing appli­ca­tions are not con­sid­ered steels because their behav­ior is sim­i­lar to oth­er types of mate­ri­als. Their com­po­si­tion is com­plex and includes impor­tant per­cent­ages of nick­el and chromi­um, with oth­er ele­ments added to pro­vide spe­cial properties.

Nick­el or Cobalt Base Alloys

Oth­er heat resist­ing mate­ri­als include alloys whose base met­al is nick­el or cobalt, while their com­po­si­tion is mod­i­fied by the addi­tion of oth­er dif­fer­ent ele­ments for spe­cial purposes.

Alloy­ing base met­als as above with var­i­ous ele­ments pro­duces dif­fer­ent class­es of mate­ri­als. Those which derive their prop­er­ties exclu­sive­ly from their com­po­si­tion and are not sus­cep­ti­ble to improve­ment by heat treat­ment are des­ig­nat­ed as hard­ened by sol­id solu­tion. Weld­ing heat-resist­ing mate­ri­als from this type is eas­i­ly per­formed. Typ­i­cal Iron base of this type is called N‑155 (or Mul­ti­met). Typ­i­cal Nick­el Base are Inconel 600 and Hastel­loy X. Typ­i­cal Cobalt base are L‑605 (or HS-25) and S‑816.

Oth­er class­es, called hard­en­able by solu­tion and pre­cip­i­ta­tion (or aging) process­es, respond to heat treat­ment because of sub­tle reac­tions that mod­i­fy their microstruc­ture, tak­ing place while heat­ing and cool­ing. Typ­i­cal Iron bases of this type are A‑286 and Incoloy 901. Typ­i­cal Nick­el base is Waspaloy.

A dif­fer­ent class of mate­ri­als, with a large set of prop­er­ties in com­mon with those dis­cussed above, are called cor­ro­sion resist­ing alloys, designed to resist attack by aggres­sive chem­i­cals, with or with­out the influ­ence of super­posed heat.

When con­sid­ered as base met­als, Nick­el and cobalt have a set of inter­est­ing prop­er­ties which make them use­ful for ele­vat­ed tem­per­a­ture appli­ca­tions because of their heat resis­tance, espe­cial­ly for gas tur­bines, fur­nace acces­sories, hot chem­i­cal pro­cess­ing sys­tems, and also for cor­ro­sion resist­ing applications.

Weld­ing Heat-Resist­ing Alloys

As a gen­er­al rule weld­ing heat-resist­ing mate­ri­als should be per­formed in their most duc­tile con­di­tion, often des­ig­nat­ed as the annealed or solu­tion-treat­ed con­di­tion.
Nick­el is a duc­tile metal­lic ele­ment used for alloy­ing steels and stain­less steels, and as such it mod­i­fies the prop­er­ties of the alloys involved. As a base met­al it is used for its remark­able resis­tance to heat and cor­ro­sion. In par­tic­u­lar it can devel­op ele­vat­ed resis­tance to stress under heat, both in cast or wrought form, when alloyed and treat­ed as needed.

Nick­el base mate­ri­als are select­ed for their cor­ro­sion resis­tance and ele­vat­ed tem­per­a­ture prop­er­ties, with ade­quate heat treat­ments. Although cov­ered by spec­i­fi­ca­tions, they are known most­ly by com­mer­cial names. Alloys hard­ened by Sol­id Solu­tion are read­i­ly weld­ed in annealed con­di­tion. Pre­cip­i­ta­tion hard­en­able alloys in wrought form are weld­ed in solu­tion treat­ed con­di­tion, fol­lowed by heat treat­ment as required. Of the weld­able wrought alloys names we can list a few here­after: Hastel­loy B, C, C276, N, X, Inconel 600, 601, 625, Rene 41.

Cobalt too is a duc­tile met­al. It is used as a major alloy­ing ele­ment for a wide selec­tion of spe­cial pur­pose mate­ri­als. As a base met­al, alloyed with oth­er ele­ments, its major prop­er­ty is the abil­i­ty to resist oxi­da­tion and scal­ing at ele­vat­ed tem­per­a­ture, although devel­op­ing only lim­it­ed strength at high temperature.

Cobalt base mate­ri­als have some­what dif­fer­ent com­po­si­tions depend­ing on whether they are in cast or wrought form. Some of the cast alloys are known by the fol­low­ing names: HS 21, X 40 (Stel­lite 31), G 34, Mar M 509, and FSX 414.

Com­mon Wrought Alloys Include: S 816, L605 (HS 25), HS 188, Mar M 918, and G 32 B. All of these have around 20% Chromi­um and some car­bide form­ing ele­ments like Nio­bi­um, Tan­ta­lum, Zir­co­ni­um, Vana­di­um. Car­bon con­tent is 0.25 – 1.0% for cast­ing alloys and 0.05 – 0.4% for wrought alloys.

Weld­ing

All major weld­ing process­es are applic­a­ble, exclud­ing pos­si­bly the oxy­acety­lene method that is not rec­om­mend­ed because the use of flux­es intro­duces com­pli­ca­tions which are not present with oth­er techniques.

Fric­tion weld­ing can be used for weld­ing heat-resist­ing mate­ri­als. For those gain­ing their prop­er­ties through solu­tion and pre­cip­i­ta­tion hard­en­ing, one must be aware of the influ­ence of the weld heat on the prop­er­ties in the imme­di­ate vicin­i­ty of the joint. If the out­come of reduced strength is not objec­tion­able, there are no oth­er lim­it­ing considerations.

Resis­tance weld­ing, both spot and seam is wide­ly used for weld­ing heat-resist­ing alloys. In par­tic­u­lar many heat resist­ing sheet met­al items, like com­buster lin­ers, flame hold­ers and many oth­er ele­ments of mod­ern gas tur­bine engines and of oth­er hot work­ing machine parts are spot and seam weld­ed as pro­duc­tion or repair pro­ce­dures much the same as is done with more cur­rent stain­less steels.

It should be not­ed that in many cas­es, arc weld­ing heat-resist­ing alloys can pro­duce crack­ing, espe­cial­ly in those hard­ened by solu­tion and pre­cip­i­ta­tion heat treat­ments, dur­ing weld­ing or dur­ing heat treat­ment: that is why much atten­tion should be paid to avoid crack­ing by devel­op­ing suit­able pro­ce­dures. All of the arc weld­ing process­es can be used, but some are bet­ter suit­ed than oth­ers depend­ing on the thick­ness of the materials.

Gas Tung­sten Arc Weld­ing heat-resist­ing alloys is best for thin sec­tions. It is good prac­tice to have fix­tures hold­ing the ele­ments fit­ted with a back up cop­per bar with tiny holes abut­ting in a groove through which a thin stream of argon is provided.

Filler met­al com­po­si­tions for weld­ing heat-resist­ing alloys should be com­pat­i­ble with that of the base met­al and of such duc­til­i­ty as to pro­vide max­i­mum free­dom from crack­ing when con­sid­er­ing the dilu­tion ratio of filler to base metal.

Before weld­ing heat-resist­ing pre­cip­i­ta­tion hard­en­able alloys, they should be relieved of all form­ing or bend­ing stress­es by a suit­able process anneal­ing heat treat­ment, pos­si­bly in a vac­u­um or con­trolled atmos­phere fur­nace in order to pre­vent oxi­da­tion. In nec­es­sary, re-solu­tion and pre­cip­i­ta­tion (aging) treat­ment should imme­di­ate­ly fol­low weld­ing. Shield­ed Met­al Arc Weld­ing is used some­times for sol­id solu­tion strength­ened heat resist­ing alloys, but is not used for solu­tion and pre­cip­i­ta­tion hard­ened ones.

Con­trol­ling Defects

Of the defects like­ly to appear in this type of weld­ing heat-resist­ing alloys, poros­i­ty is con­trolled by prop­er clean­ing before weld­ing and remov­ing of sur­face con­t­a­m­i­na­tion. Clean­ing the work­piece and the filler met­al before weld­ing heat-resist­ing alloys is crit­i­cal. Cracks of any type, in the weld or in the base met­al are nev­er admit­ted. Joint design should avoid stress con­cen­tra­tion and mul­ti-axi­al stress­es. High heat input pro­duc­ing large resid­ual shrink­age stress­es can also be a cause for cracks.

High ener­gy weld­ing of heat-resist­ing alloys presents var­i­ous lev­els of weld­abil­i­ty depend­ing on the mea­sure of restraint the parts under­go dur­ing weld­ing. Even nick­el cast alloys, which nor­mal­ly exhib­it very low weld­abil­i­ty, can be elec­tron beam weld­ed for non demand­ing applications.

If you are inter­est­ed in automat­ing a nick­el or cobalt weld­ing appli­ca­tion, con­tact our staff at 877−762−6881 or fill out a con­tact form and our engi­neers can work out the best options for your process requirements.