tips for successfully welding stainless steel to carbon steel

8/10/2019 Tips for Successfully Welding Stainless Steel to Carbon Steel

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Tips for Successfully Welding Stainless Steel to Carbon Steel

Ideally, the base materials used in a welding application will match perfectly in chemical and

mechanical composition. However, companies in manufacturing, fabricating, construction and

other industries may occasionally find it necessaryboth for cost and service conditionrequirementsto weld together dissimilar materials. Welding differing materials, such as

stainless steel and carbon steel, can be a much more economical choice than fabricating a

structure entirely with stainless steel.

The welding of dissimilar materials is common in certain power generation facilities such

aspetrochemical refineries, as well as in many mining and mineral processing facilities. The

corrosion resistance provided by stainless steel is often necessary for equipment in those

facilities. When the environment or service conditions permit, however, the material can bewelded to less expensive carbon steel. In these applications, carbon steel, which includes mild

and low alloys, serves the important role of reducing costs in constructing and operating thesemanufacturing plants.

As with any welding application, attaining success when welding dissimilar steels requires thecareful selection of filler metals and the proper welding procedures. This holds true regardless of

which process the welding application uses.

Note, the topic of joining dissimilar metal spans an enormous number of materials and

fabrication processes. The advice and suggestions offered in this article apply to a range of

stainless steel and carbon steel, including the commonly used 304L austenitic stainless steel andmild steel combination, as well as mentions of duplex and other such stainless steel grades.

Welding operators who are uncertain about an application should always consult with a welding

distributor or a filler metal manufacturer for specific welding and filler metal recommendations.

Three factors to rememberWhen welding stainless steel to carbon steel, it is critical to pay attention to chemistry,mechanical properties and corrosion resistance to avoid potential trouble. For all three factors,

choosing the right filler metal can help reduce concerns.

As an example, when joining 304L stainless steel to mild steel, the most commonly

recommended filler metal is 309L. During the welding process, the weld becomes diluted withsome of the stainless steel from one side of the joint and some of the mild steel from the other

side, mixing in material from each side of the weld. The goal is to create a final weld depositwhose chemistry is compatible with each side of the weld joint. Using 309L filler metal achieves

this goal when joining 304L stainless steel to mild steel.

Again, if there is uncertainty about the proper filler metal selection, remember to consult with a

welding distributor or a filler metal manufacturer prior to attempting the dissimilar weld.


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Matching the mechanical properties of each type of material is important, as well. Attaining a

mechanical match is a function of having the correct chemistry, and also a reflection of the heat

created by the welding procedure. As a general rule, when welding any type of stainless steel tocarbon steel, the filler metal should match or slightly exceed the mechanical properties of the

weaker of the two materials.

Lastly, it is important when welding stainless steel and mild steel to maintain the corrosion

resistance of the weld joint and the nearby stainless steel base metal.

Heat input is importantTo address the factors of chemistry, mechanical properties and corrosion resistance, it is

important to follow a suitable welding procedure that limits the heat input to the weld and

stainless base material. Limiting the heat input reduces dilution of the weld deposit with the mild

steel portion of the weld joint. This in turn helps maintain the alloy content of the weld depositand its desired corrosion resistance.

In the case of some stainless steels, moderate heat input also protects the corrosion resistancebyavoiding the creation of undesirable phases in the stainless steel side of the joint. For example,

300 series austenitic stainless steels are subject to carbide precipitation if held too long in a

critical temperature range of 800 degrees to 1400 degrees Fahrenheit. Minimizing the time in thisrangeand selecting a low carbon base metal and filler metalcan prevent this problem from

occurring. The use of stabilized grades of filler metals (ER321 or ER347, for example) may also

be warranted, and can serve as added insurance to avoid carbide precipitation.

Other stainless steel grades may form undesirable phases that result in brittleness or poor

corrosion resistance if held too long at a high temperature. Sigma phase (a brittle, intermetallicphase with high hardness) can form in some stainless grades at elevated temperatures and can

seriously compromise mechanical and corrosion resistance properties. In duplex stainless steels,for example, the heat input is responsible for the balance between ferrite and austenite in the

final weld and heat-affected zone (HAZ). The proper level of heat input can help to maintain the

desired amount of each phase in the finished weld and base metal HAZ.

Pitfalls to avoid: warping, cracking and oxidationStainless steel has a high coefficient of thermal expansion, a measure that refers to the rate at

which a material expands with changes in temperature. In short, stainless steel expands and

contracts more with temperature changes in comparison to carbon steel.

Stainless steel also has about half the thermal conductivity of carbon steel. Because of that lack

of thermal conductivity, a piece of hot stainless steel will remain hot for much longer because itdoesnt conduct heat away from the source as quickly. Since carbon steel has more thermalconductivity, heat conducts along that piece relatively quickly, drawing heat away from the weld

zone.

Differences in the coefficient of thermal expansion and the thermal conductivity can cause some

difficulty when welding dissimilar materials. Stainless steel will naturally want to expand and

contract more as a result of the high heat seen during welding. Conversely, carbon steel


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(particularly mild steel) is a good conductor of heat and therefore will cool more rapidly and

shrink faster as the joint cools. These differences add to the stress on the joint, created as both

sides expand with heat and contract with cooling. This can cause warping or misalignment of adissimilar metal weld. It can also cause cracking if the stresses created by differences in thermal

expansion and contraction exceed the strength of either material.

To address these two issues when welding stainless steel to carbon steel, avoid highly restrained

joints that create high stresses as the joint is heated and cools down. If a highly restrained joint

configuration is required, use modest heat input and some pre-heating to delay cooling of thejoint after welding is completed. Insulating the weld joint after the last weld pass also will slow

cooling and help prevent thermal stresses from cracking a joint.

Contamination of the weld joint and resulting weld is a serious problem that often causes hot

cracking. Contaminants can react with the carbon steel or stainless steel to produce minute

amounts of weld material with drastically lower melting temperatures. These microscopic areas

of low-melting-point contaminants are the last to freeze as the weld cools. As a result, they can

become cracks as the weld metal cools and shrinks. These hot cracks can be easily seen if theproblem is severe, but they can also be micro-cracks that are invisible to the naked eye.

When it comes to protecting the material from oxidation, these dissimilar metal welds should be

treated just as a stainless weld would. Open root joints should be shielded from the atmosphere

on the backside of the weld (back purging). The practice of back purging, most frequently usedwhen TIG welding, helps prevent contamination of the weld from behind the joint. Otherwise,

the weld joint and stainless steel side of the weld can be damaged by oxidation, which is the

result of a reaction with oxygen and nitrogen in the atmosphere. Oxidation will damage the

corrosion resistance of the weld and stainless steel HAZ. To prevent this from occurring, purgethe back of the joint with an inert gas such as argon, or use one of the commercially available

coatings that can be applied to the back of a weld joint prior to welding.

Preparation for stainless steel to carbon steel weldingProper cleaning and preparation are vital steps to help ensure successful welding of dissimilar

materials. Grind the mill scale or coatings back by at least 1/2 inch on each side of the joint.Follow this task by cleaning the area with a solvent such as alcohol or acetone. These steps help

get rid of grease and oil, which tend to carry the phosphorous and sulfur that are the primary

causes of hot cracking.

Welding dissimilar metals requires planningWelding dissimilar metals can be a challenge. It is important to have as much information aspossible about the characteristics of the base materials and the filler metal in order to make

appropriate choices that yield successful welds. When in doubt, consult with a trusted welding

distributor for advice on the process. Doing so can help ensure the durabilityand the cost-

savingssought in an application featuring both stainless steel and carbon steel.


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5.11 WELDING DISSIMILAR STEELS Stainless steels are expensive and the higher the alloy

content of the steel, the higher the cost. The most efficient design of a structure calls for the use

of the higher alloy steels only where they are needed. Such a design may call for severaldifferent steels to be used. As mentioned above, there is no problem of electrode selection when

welding stainless steels or any steel to a steel of the same type. Simply match the electrode to

the steels. When a change from one type of steel to another (called a transition weld) is made,care must be given to the selection of the electrode used. 5.11.0.1 There are two generalconditions and rules for electrode selection to weld dissimilar steels. a. When the steels are

similar metallurgically but dissimilar chemically, match the electrode to the lower chemical

composition or less expensive steel. For example, type 310 steel (25% chromium, 20% nickel) issometimes welded to type 304 steel (19% chromium, 10% nickel). Both types are austenitic.

Type 304 steel, which is welded with 308 electrodes, is less expensive, so that weld would be

made with type 308 electrodes rather than type 310 electrodes. b. When the steels to be jointed

are different metallurgically and chemically, the electrode is selected to provide a tough, crackresistant weld between the two steels. For example, 304 stainless steel is frequently welded to

mild structural steel. Corrosion resistance cannot be part of the problem because mild steel is on

one side of the joint with practically no corrosion resistance compared to the stainless steel. Ifthis weld is made with mild steel electrodes to match the mild steel side of joint, the weld metal

would be enriched by the wash- in of chromium and nickel from the stainless side. This

intermediate chrome-nickel is usually hard and crack sensitive. If the weld is made with type

308 electrodes to match the stainless steel side of the joint, the chromium and nickel contents ofthe weldment are diluted by the mild steel side of the joint to an intermediate level that would

again probably be hard and crack sensitive. When welding mild steel to stainless steel, a

proportion of 18% chromium and 8% nickel is desirable in the weld deposit to produce soundwelds, with 17% chromium and 7% nickel being the minimum allowable amounts. 5.11.0.2 The

following examples in Figure 14 show the results of making a transition weld of mild steel to

304 stainless steel with three different electrodes.

5.11.0.3 Normally the most severe dilution of the weld metal by the base metal is 40%. Thus,

the weld metal in the joint is comprised of 60% from the electrode and 40% from the base metal

as shown in Figure 14. In the case of butt joints between dissimilar steels, half of the dilutioncomes from each side of the joint, or 20% from each base metal.

5.11.0.4 Many times, type 310 and 312 electrodes are used erroneously for welding stainless to

mild or low alloy steel. In many cases, not only can more dependable welds be made with 309

electrodes, but appreciable savings can be achieved because of their lower cost.

5.11.0.5 Another common use of stainless steel filler metals is the overlaying or cladding of less

expensive steels with a layer of stainless. Mild steel tanks designed to hold corrosive liquids

may be lined with stainless steel in this manner. Usually, continuous bare or flux coredelectrodes are used with an automated welding setup. Current and penetration must be

controlled closely to limit dilution with the base metal. Sometimes it is necessary to deposit

more than one layer to assure the correct analysis of the deposit. 5.11.0.6 The welding ofstainless clad plate (produced by some steel mills) should also be mentioned. Thicker sections

may be welded with both mild steel and stainless electrodes, and thinner sections may be welded

only with stainless electrodes. Joint preparation, welding procedure and electrode selection will


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vary with the thickness and type of clad plate being welded. Welding of clad plate is a

specialized area of dissimilar metal welding and beyond the scope of this course.

tips for successfully welding stainless steel to carbon steel

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