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Post-Weld Stress Relief and Filler Metal Selection

Welding Design
Tue, 2009-09-01 12:00
What does "repair" mean to a welder? There are a series of factors to consider and issues to ascertain
before planning any repair project.
Q: I need to repair a shaft made of AISI 4340. I need to weld an AISI 4130 shaft to a 1018 pipe. I
need to fix a crack on an AISI 4140 plate. Which electrode should I use?
A: These are very frequent questions that typically cannot be answered without fully considering the
application. For example, what does repair mean in the question above? Does it mean the shaft is
worn down and needs to be built back up? Or is the shaft broken in half and need to be put back
together? All of these are important issues when considering weld repair and selecting welding
consumables. The intent of this column is to simplify the important points of this very complex
subject and present some typical answers for the most common materials that most experienced
welders can follow. Let's start with a basic understanding of this family of high-strength steels and
then go into welding repair.
The medium-carbon, low-alloy, high-strength steels include (but are not limited to) AI SI 4130, 4140,
and 4340 steels. These materials have a good strength-to-weight ratio, good ductility and toughness,
and high hardenability. Components made of these steels won't wear as much as a low-carbon
materials (1018 steel), and won't break as readily as high-carbon materials (1055 steel.) Also, AI SI
low-alloy steels do not readily lose strength at elevated temperatures, which makes them excellent
steels for parts like shafts, pinions gears, etc.
These materials attain their good mechanical properties through a process called quenching and
tempering. I n quenching and tempering, the steel manufacturer heats the steel above a critical point
(typically greater than 850C or 1,560F), then cools the material very quickly (quenching) to create
martensite (a very hard, brittle but strong microstructure), and then re-heats the material to soften
(reduce hardness and strength) and add ductility and toughness (tempering.) Figure 1 shows a
typical quench and temper cycle.
The key points to understand about the quench and temper process are:
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Critical cooling rate must be exceeded to produce 100% martensite.
As-quenched hardness is determined by carbon content (more carbon, higher hardness;
Rockwell C scale.)
Now, let's address what happens when you weld on these materials. Figure 2 shows that the red area
directly under the weld bead is a zone that gets heated above the critical temperature (greater than
850C/ 1560F), and then gets cooled very quickly to the base metal temperature (in this case room
temperature.)
As we learned above, what's called quenching and tempering produces a hard, brittle microstructure
called martensite that readily cracks. For this reason, welding on AI SI 4130, 4140, and 4340 without
preheat is not recommended, no matter what filler metal is used. Preheat slows down the cooling
rate and helps prevent quenching in the HAZ, and thus helps prevent martensite in the base metal.
Table 1 shows recommended preheat/ interpass temperatures for many AI SI low-alloy steels.
The key points to remember about preheating are:
Preheat slows down the cooling rate and prevents quenching that creates martensite in the
HAZ.
The amount of preheat is determined by the amount of carbon and other alloys in the material
(more carbon and alloy requires more preheat; see Table 1.)
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The heat from welding makes the base material expand during heating and contract when cooling.
This creates residual tensile stresses around the weld that can cause cracking or shorten the life of
the part. Typically, AI SI 4130, 4140, and 4340 require a stress relief after welding to eliminate these
residual stresses. Figure 3 shows a typical post-weld stress relief cycle where the weldment is heated
to a typical temperature of 650C (1,200F) and held for 1 hour per inch of thickness.
The key points to understand about stress relief are:
Stress relief is recommended after welding AI SI alloy steels, especially for steels with carbon
contents higher than 40% (eg. 4140 or 4340.)
Stress relief cycle immediately after welding promotes hydrogen diffusion out of weld metal
and HAZ to help prevent hydrogen cracking.
Sometimes, with the proper preheat, 4130 can be cooled in still air after most arc
welding/ stress relief processes (use an insulated welding blanket, or bury in sand/ vermiculite
as a precaution.)
For 4140 and 4340, cooling in a controlled furnace (or using an insulated welding blanket, or
burying in sand/ vermiculite) is required to slow the cooling rate down to below 65C (150F)
per hour. Use temperature crayons to check regularly.
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I n all cases, cooling in still air is acceptable once the weldment is below 93C (200F.)
Selection of filler metals requires a decision about what function the weld repair has to perform.
Does the weld/ weldment need to be hard like the parent material to resist abrasion/ wear? Does the
weld/ weldment have to be ductile so that is does not break/ crack easily. Does it have to be as strong
as the base material? Regardless of the answers to these questions, the welding process and/ or filler
metal must not introduce hydrogen into the weld metal and HAZ. Low-hydrogen processes include:
properly stored, low-hydrogen SMAW electrodes classified less than H4; gas-shielded GMAW and
GTAW processes classified less than H4; and SAW processes with wire/ flux combinations classified
less than H4. Below, we will address each process, commonly recommended filler metals that match
the base metal, and commonly recommended filler metals that do not match the base metal.
Matching When you must closely match the composition of the base metal, Table 2 lists
commonly used, covered electrodes for SMAW and wires for GMAW/ GTAW. Typically, carbon
contents are lower for matching filler metals, to prevent the creation of hard, brittle martensite in the
weld metal.
Non-Matching There are cases where you do not have to match the strength of the base material.
Typically, low-strength filler metals are selected to ensure a more ductile weld. Keep in mind that
depending on the amount of dilution (high dilution means that alloy in the base metal is melted into
the weld metal), the resulting weld metal will be stronger due to pickup of carbon and alloys from the
base metal. Here are some examples of cases where non-matching base metals can be selected.
Welding AI SI low-alloy to low-carbon steel: When welding AI SI 4130 to AI SI 1018 carbon
steel, you only need to match the strength of the 1018 steel: E7018-A1 or ER70S-6 filler may
suffice.
Fillet welding: Often, a lower-strength filler metal can be used when you have the latitude to
produce a larger fillet weld. For example, welding 4340 to 4340 may require a one-quarter-
inch fillet weld using filler metal with matching strength. But, you may be able to use a lower-
strength filler and make a larger (5/ 16 or 3/ 8 in.) fillet weld to achieve the strength needed for
the weldment.
Building up shafts: Shafts that wear typically require buildup and re-machining. I n this case,
the original shaft can be machined down to allow build-up using a low-strength filler metal,
followed by a hard-facing layer. Adding a lower strength build-up layer helps prevent cracking
in the shaft as well as under the hard-facing layer.
The key points to understand about filler metal selection are:
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Carefully consider the need for the weld metal to match the base material composition. Using
a lower-strength filler metal may result in a weldment that is less susceptible to cracking.
Whether or not the weld metal matches the base metal, for AI SI alloy steels preheat and
interpass temperature management applies as well as post-weld cooling rate management.
Ken Lee, Application Engineer at Lincoln Electric Co., is a Certified Welding Engineer who has
been with Lincoln Electric for 19 years. Kevin Beardsley, Application Engineer at Lincoln
Electric, is a registered Professional Engineer with 21 years of manufacturing experience.
Address your welding questions to "Welding Q&A" at WDeditor@penton.com
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