Welding metals of dissimilar strengths
Successfully welding such material requires the right filler metal and joint design
By Dean C. Phillips
n an ideal world all metals would match perfectly,
both in their mechanical and chemical composition.
It would certainly simplify welding, especially when
selecting filler metals. Yet in many applications it is necessary to weld materials with different strengths. By
doing so you can reduce material costs and use metals
that are better-suited to the end-service conditions of
the completed component.
Beyond just using the correct filler metal, successfully
welding dissimilar-strength steels requires close attention
to a few other factors. You must know the weldability of
the materials to be joined; assess the service condition to
which the parts will be subjected; and use the correct
welding, preheat, and interpass temperatures for the application.
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Making a Good Match
In most cases, you should match the tensile strength of
the filler metal and the lower-strength material as closely
as possible. Doing so offers the least potential for cracking. For example, when welding A514—a minimum 110-
KSI tensile, low-alloy, quench-and-tempered steel—to
A36 steel with a minimum 58-KSI tensile strength,
choose a filler that more closely matches the A36 base
metal, such as a filler metal with 70-KSI tensile, the lowest strength typically available on the market.
Certain joint designs may allow you to undermatch
the filler metal’s strength to the lower-strength material.
For instance, some fillet joints created by joining a 100-KSI
yield, quench-and-tempered material like A514 to an
even higher-strength, proprietary high-strength, low-alloy
130 material can accommodate welds with 70-KSI filler
metals. Despite the lower-strength weld metal, the joint
design in this situation still provides adequate overall
strength for the application. You always should consult
the welding specifications to determine whether under-
matching the filler metal is appropriate.
Weldability
Weldability refers to the general ability of two materials
to be joined successfully without defects like cracking
from material chemistry issues, while also obtaining the
mechanical properties necessary for the application.
Weldability varies from material to material and often
depends on other criteria, such as the joint configuration and the service conditions the final weldment will
encounter.
Determining weldabilty helps establish how easily dissimilar materials can be welded, as well as which welding
process and filler metal are most appropriate for the job.
And it also helps determine how suitable the materials
are for the application. For example, if welded components will be under a cyclic load, the two dissimilar-strength materials and the deposited weld metal must be
compatible for fatigue life.
Part of determining weldability is calculating the carbon equivalency (CE) of the base materials, often using
the following CE formula: CE = C + (Mn + Si) / 6 + (Cr +
Mo + V) / 5 + (N i+ Cu) / 15. The CE helps you determine
cracking susceptibility as well as the necessary preheat
and interpass temperature controls.
To determine weldability, you need to know the
chemistry of the two dissimilar-strength base metals, as it
is probable (but not absolute) that they will have differ-
ent chemical compositions. A material’s weldability may
be identified by ASTM, ASME, AISI, SAE, or similar stan-
dards. Some standards specify the chemistry properties
required for certain material grades; other standards give
mechanical property requirements; and some standards
give both. For instance, you may need to weld ASTM
A572 Grade 50 to AISI/SAE 4140, in which the former ma-
terial has both chemistry and mechanical requirements,
but the latter has only a chemistry requirement.
Service Conditions
When welding dissimilar metals, you need to consider
the final service condition that the weldment will encounter. In certain applications, you may weld dissimilar-strength materials to create components that will
encounter high temperatures. For instance, you may
need to weld ASTM A387 chrome-moly steel tubes—for
a high-temperature, corrosion-resistant boiler application—to a transition piece of A36 steel that will not encounter the same temperatures.
In this situation, the chrome-moly may have a yield
strength around 60 KSI and a tensile strength of 80 KSI;
the A36 steel has a 36-KSI yield strength. Not only do you
need to select a filler metal that matches the lower-strength material, but you also may need to find a filler
metal capable of being stress-relieved. High-temperature
materials (in this case, the chrome-moly portion) typically don’t require the toughness brought forth by stress-relieving. But because this application uses A36 steel, the
weld and A36 base metal may still require PWHT depending on the design application.
