THE MEANING OF WELD REPAIR

What does "repair" mean to a welder? There are a series of factors to consider and issues to ascertain before planning any repair project. For example, assume that I need to repair a shaft made of AISI 4340, I need to weld an AISI 4130 shaft to a 1018 pipe, and I need to fix a crack on an AISI 4140 plate. Which electrode should I use?

Typically, this question cannot be answered without fully considering the application. In this case, what does ?repair? mean in the example 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) AISI 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, AISI 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. In quenching and tempering, the steel manufacturer heats the steel above a critical point (typically greater than 850 deg C or 1,560 deg F), 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) (see Figure 1). The key points to understand about the quench and temper process are:
? Critical cooling rate must be exceeded to produce 100 percent 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 850 deg C/1560 deg F), 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 AISI 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 AISI 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).

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, AISI 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 650 deg C (1,200 deg F) and held for one hour per inch of thickness. The key points to understand about stress relief are:
? Stress relief is recommended after welding AISI alloy steels, especially for steels with carbon contents higher than 40 percent (e.g., 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 65 deg C (150 deg F) per hour. Use temperature crayons to check regularly.
? In all cases, cooling in still air is acceptable once the weldment is below 93 deg C (200 deg F).

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 AISI low-alloy to low-carbon steel: When welding AISI 4130 to AISI 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 in 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. In 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:
? 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 AISI alloy steels preheat and interpass temperature management applies as well as post-weld cooling rate management.

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Kevin Beardsley, a registered professional engineer with 21 years of manufacturing experience, is an application engineer at Lincoln Electric , 22801 St. Clair Avenue, Cleveland, OH 44117-1199, www.lincolnelectric.com. For questions or comments on this column, contact Kevin at 216-383-2259 or [email protected].