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718 Nickel to 316 Stainless Dissimilar Metal Welding 

by Kristopher Doll

Abstract

This project investigates the technical considerations of dissimilar metal welding between nickel alloy

718 and stainless steel 316 using the GTAW process. A brief review of concerns with welding 718, 316,

and creating dissimilar metal welds is presented. Experiments are performed to evaluate heat affected

zone strength, effective joint tensile strength, and the effect of groove design on penetration depth.

Base and Filler Metal Properties

The materials involved in this project are 316 stainless, 718 Inconel, and ERNICrMo-3 filler rod (625

inconel). 316 is an austenitic stainless steel intended for use in corrosive environments. 718 is a group

D nickel alloy that can be precipitation hardened to 200 ksi and maintain its strength without overaging

at temperatures up to 1200⁰F. The Nickel Development Institute (NiDI) recommends using nickel alloy

625 filler rod for joining 316 to 718 [1]. 625 is a group B solid solution strengthened nickel alloy. Table 1

lists the properties and compositions of each alloy involved in this study.

Table 1: Properties of Base Metals and Filler Metal

Stainless 316L [2]

(Annealed)

Nickel 625 [3]

(As Welded)

Nickel 718 [4]

(Solutionized)

Primary means of

Strengthening

Cold Work Solid Solution

Strengthening

Precipitation

Hardening

UTS (ksi) 80 120 130

Yield (ksi) 35 80 58

Elongation 60% 17% 45%

Service Temperature (F) 800 1500 1200

Composition Ni 10-14% Ni 58% Ni 50-55%

Cr 16-18% Cr 20-23% Cr 17-21%

Mo 2-3% Mo 8-10% Mo 2.8-3.3%

Fe 62-72% Nb 3-4% Nb 4.8-5.5%

C .03-.08% Fe Balance

Ti 0.7-1.2%Al 0.2-0.3%

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Concerns When Welding 316 Stainless

There are two possible problems that may occur in the heat affected zone (HAZ) of an austenitic

stainless steel such as 316:

1) 

High temperatures will remove the strength gained by cold work and will increase grain size.

These two effects will reduce the tensile and yield strength of a cold worked material and

increase its ductility.

2) 

In the 800-1600°F temperature range, chromium carbide precipitates out of solution near grain

boundaries. This process of sensitization depletes the chromium available to form an oxide

layer and greatly reduces the corrosion resistance in the HAZ. [5]

Both problems can be minimized by welding quickly and promoting rapid cooling. For applications that

utilize the corrosion resistance of 316, the reduction in strength is not particularly problematic.

Sensitization can be avoided by using Extra Low Carbon (ELC) grades of 316 or stabilized grades that

have carbide forming elements. In standard 316 stainless grades, the chromium carbides can be

dissolved with a 1900°F anneal and quench treatment.

Concerns When Welding 718 Stainless

The HAZ of 718 may experience the following problems [6]:

1) 

Microfissures can appear where the grain boundaries melt and crack upon cooling. This is more

of an issue when the material previously underwent a high temperature solution anneal and the

welding cooling rates are very fast.

2) 

Laves phase intermetallic compounds may precipitate out of solution with slow cooling rates.These compounds severely decrease the ductility of the weldment.

The strategy for avoiding both of these issues is to weld with a moderate heat input and low interpass

temperature. Microfissures cannot be removed except by repair welding the HAZ. Intermetallics can be

mostly dissolved with a high temperature solution anneal after welding.

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Weld Metal Concerns

With a dissimilar metal weld, the weld metal will be diluted by the base metals. Dilution in a manual

GTAW is highly dependent on welding technique and typically ranges from 20 to 50% [7]. The resulting

composition of the fusion zone can be determined through chemical analysis or it can be estimated by

measuring the cross sectional area of the weld bead and the melted base metal (figure 1). Due to the

high viscosity of nickel, the weld metal composition may vary rather drastically across a weld bead.

Dilution can be reduced by minimizing the amount of base metal melted; however, this may lead to

reduced root penetration.

Base metal dilution will give the weld inferior mechanical properties

and corrosion resistance compared to the filler and base metal

properties. Furthermore, it is possible that tramp elements in any of

the three metals may combine to form intermetallic compounds.

Using the 20-50% dilution figure and assuming perfect mixing, the

composition of the weld can be approximated as shown in table 2.

Special Metals Corporation reports a transverse dissimilar metal weld

tensile strength of 92 ksi for 304 joined with 625 filler and 107 ksi for

718 welded with 625 [3]. These strengths are inferior to the 120 ksi all

weld metal tensile strength of nickel 625.

Groove Geometry for Nickel Alloys

Nickel alloys have a high molten viscosity and relatively low thermal conductivity. Welders describe

these alloys as being very “sluggish”. Achieving moderate root penetration can be a challenge.

A study by J. Gordine concluded that the optimal V-groove included angle for welding 718 with matching

filler is 90 degrees. Furthermore, the study found that root penetration appears to increase linearly

with root gap. [6]

Table 2: Approximate 316

to 718 Weld Composition

with Nickel 625 Filler

20%

Dilution

50%

Dilution

Ni 53 45

Cr 21 20

Mo 7.8 5.9

Nb 3.3 3.0

Fe 8.8 22

Ti 0.1 0.3

Figure 1: Base Metal Dilution Cross Sectional Area Method Taken from [7] 

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Welding Parameters for Manual GTAW

Table 3 lists the recommended current and travel

speed for welding 1/2" plates of 316 and 718 with

GTAW. There is a large mismatch between these

values. With a nickel based filler metal, the weldzone will behave more like a nickel alloy than a

stainless steel, and the actual welding parameters

should be closer to those of 718.

Helium shielding is recommended for nickel alloys since it facilitates deeper penetration. However,

helium gas was not available for these experiences.

The actual welding procedure involved 150 amps maximum current, DCEN polarity with a 3/32” 2%

thoriated tungsten electrode, argon shielding, 3/32” ERNiCrMo-3 filler rod, and a travel speed of 1 to 6

inches per minute. Samples were cut from 1/2" diameter round stock.

The samples were placed together and the root gap was established using a set of machinist shims. Two

tack welds were created on either side of the sample. The solidification shrinkage of the tack welds will

cause the actual root gap to be somewhat smaller. The root pass and subsequent passes were made

using a slight weaving motion. Care was taken to gradually decrease the current at the end of a weld

pass. The black nickel oxide was removed with a wire brush after each pass. A low interpass

temperature was maintained by quenching the sample in water whenever the weld glowed red

following a welding pass.

Preliminary Experiments

A series of short experiments were conducted to determine V groove angles and root separation for

achieving sufficient root penetration.

It was determined from these experiments that weld beads created on a workpiece with a groove angle

of 60 degrees or smaller is incapable of reaching the root of the groove for root openings of .03” and

less. A V groove angle of 90 degrees appears sufficient for melting the metal near the root of the

groove. A wider groove would be unnecessary. 

Table 3: Recommended Welding Parameters

Base Metal 316 [5] 718 [8]

Nominal Amperage 200-300 120Travel Speed (ipm) 10-12 3

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Heat Affected Zone Survey

A single weld bead was created joining two pieces of 1/2" diameter round stock. One piece was 316

stainless and the other nickel 718. The hardness was measured at .1” increments across the weld and

heat affected zones with an HRA test. Figure 2 shows the hardness survey with the approximate UTSvalues converted from the HRA values.

The average HRA of the samples before welding is 62.0 for Nickel 718 and 62.6 for Stainless 316. The

Nickel 718 was in the solutionized condition and the 316 was received cold rolled from the vendor. A

number of trends are evident from the plot:

  The Stainless 316 strength decreases near the weld zone due to annealing and grain growth.

  The Nickel 718 increases in strength in the HAZ due to aging. The test point closest to the weld

did not change in hardness; the temperature it reached and subsequent cooling rate caused it to

return to a solutionized condition.

 

The weld zone has substantially lower strength compared to the reported tensile strength of

120 ksi for an all-weld-metal nickel 625 sample. This is the result of base metal dilution.

Furthermore, the strength is lowest near the stainless 316 base metal; this region of the weld

may have a higher concentration of iron dilution

Figure 2: HRA Hardness Survey

Nickel 718  Stainless 316

 

Weld

Zone

(Nickel

625) 

177 

138 

108 

88 

75 

 A  p p r  o x  i   m a t   e U T  S  (   k  s  i   )  

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Transverse Weld Tensile Test

A tensile test specimen was created by

welding two pieces of .75” 316

stainless round stock on either end of

a .5” diameter nickel 718 bar. Aqualifying transverse tensile specimen

for a code weld would typically be

machined out of a large test weld

deposit. However, the tensile

specimen created for this project was

designed to minimize labor and

material expense.

The ends were turned on a CNC

machine to the appropriate geometry:

a 90° V with a .1” root diameter. The

root gap was approximately .050”.

The samples were tack welded

together with 625 filler and fully welded with several weld passes.

The specimen was mounted in a turning center and the machined to

a gage diameter of .330”. Figure 3 shows the steps in the creation of

the tensile specimen.

Gage region marks were drawn on the sample, and it was tested at

an extension rate of .5 inches per minute. The tensile strength was

91.6 ksi and the area reduction was approximately 31% (determined

by measuring the diameter at the fracture location). The specimen

broke in the weld along a 45° angle relative to the extension

direction. An examination of the fractured end reveals a small

region of incomplete fusion (figure 4); this may account for the

failure at the weld. However, the weld strength is on par with the

reported 92 ksi tensile strength of a stainless 304 to nickel 625

dissimilar metal weld with nickel 625 filler. The specimen appeared

to neck at both welds but did not plastically deform in the 718 region; this is consistent with the yield

strength of aged 718. Since an extensometer was not used, instantaneous values of strain could not be

measured. Elastic modulus, yield strength, and ductility could not be determined from this test because

the combination of multiple metals causes non-uniform stress distribution and elongation.

Figure 3: Creation of Tensile Specimen

Figure 4: Incomplete Fusion

in Fracture Region

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Groove Design Experiment

The purpose of this experiment is to evaluate the effect of groove geometry and root opening on root

penetration. To do this, six sets of .1” deep grooves were machined on a CNC turning center: three sets

were given a 90⁰ V and the other three sets were given a U shape with a constant .1” radius. 

The samples were welded together, cut open with wire EDM, and one side was sanded to a fine finish.

Figure 5 shows the welded sample exterior and the interior revealed by EDM with the oxide layer intact.

The difference in color of the oxides offers some indication of the extent of the weld zone.

To determine the depth of root penetration, the original groove geometry was superimposed on an

image of the welded samples using CAD software and the distance from the base of the weld to the root

was measured (figure 6). Root gap was also determined using this technique.

U Groove Samples

90⁰ V Groove Samples 

Figure 6: Welded Groove Samples with Superimposed Groove Geometry

Figure 5: Welded Groove Sample Exterior and Interior Views

718 

718 

316 

316 

718 

718 

316 

316 

Increasing Root Gap 

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The measured root penetrations are

shown in figure 7. As expected,

there is a general trend of

increasing penetration with

increasing root gap. However,

there is great variance in

penetration due to welding

technique.

For small root gaps, a U groove

resulted in the greater penetration.

The reason for this is that a V

groove makes it difficult to focus

the welding arc at the root when

both sides of the groove are close

together. In fact, one of the zeroroot gap V groove tests had

incomplete fusion near the root.

For larger gaps, there does not appear to be a significant penetration difference between U and V

groove geometry. In these cases, a V groove would be desirable since it requires less filler metal and is

easier to create.

Conclusions

The hardness survey revealed how the weld zone strength is reduced as a result of base metal dilution.

In addition, the HAZ of solutionized 718 will increase in strength from aging, and the HAZ of cold worked

316 will be annealed.

The transverse weld tensile test indicated that a 316 to 718 dissimilar metal weld with nickel 625 filler

using a GTAW process can be expected to have an effective tensile strength of at least 91ksi with a

ductile failure mode.

The groove geometry tests showed how the root penetration is generally increases with root gap and is

very sensitive to welding technique. U grooves outperform V grooves for narrow root openings but

present no advantage in terms of root penetration when the opening is sufficiently large.

Figure7: Plot of Root Penetration vs. Root Opening

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 0.01 0.02 0.03 0.04

   R   o   o   t   P   e   n   e

   t   r   a   t   i   o   n

    (   i   n    )

Root Gap (in)

90° Chamfer U Groove

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References

[1] Guidelines for the welded fabrication of nickel alloys for corrosion-resistant service. Nickel

Development Institute. N11012, 1994

[2] 316 Stainless Steel, Annealed Bar. www.matweb.com. Accessed May 2014.

[3] Inconel Alloy 625. Special Metals Corporation. 2013

[4] Inconel Alloy 718. Special Metals Corporation. 2007

[5] Stainless Steels Welding Guide. The Lincoln Electric Company. 2003

[6] Gordine, J. Some Problems in Welding Inconel 718. AWS: Welding Research Supplement. 1971

[7] Avery, R. E. Pay attention to dissimilar-metal welds: Guidelines for welding dissimilar metals.

Chemical Engineering Progress. AIChE. 1991

[8] Gordine, J. Welding of Inconel 718. AWS: Welding Research Supplement. 1970