This column is sponsored by Penton and the Lincoln Electric Co., Cleveland. Dave Barton is a senior welding engineer in the Application Engineering Group of The Lincoln Electric Co. He oversees welding procedure development for both new technology and existing products, performs failure analyses for customers, and serves as a consultant on welding application problems. Barton has been with Lincoln Electric for 21 year. Send your questions for Mr. Barton in care of WDF by e-mail to: firstname.lastname@example.org
Q: How to I weld Greek Ascoloy 418 material to itself. Can you supply a filler metal recommendation? Are there any special precautions?
A: The Greek Ascoloy 418 material is a martensitic stainless for service at temperatures up to 1,200 degree F and greater creep strength than other 12 percent Cr stainless materials at higher temperatures. Ascoloy recommends using a 310 stainless filler metal to weld this alloy due to the potential service temperatures these weldments could see in use. A 400 F preheat and interpass temperature is also recommended.
Q: Our shop wants to weld an austenitic stainless steel to a piece of mild low carbon steel using a mild steel filler metal with the gas-metal arc (GMAW) welding process. Do you foresee any potential problems?
A: Yes, I do. If you use GMAW to weld mild steel to stainless using mild steel filler metal, the weld will pick up enough dilution from the stainless to become extremely hardenable and therefore brittle. Here is the reason: If the stainless base metal is 18 percent Cr, 9 percent Ni (304), and the weld produces a typical 30 percent dilution (15 percent from the stainless and 15 percent from the mild steel) the weld metal root pass will contain 2.7 percent Cr and 1.35 percent Ni, along with about 0.1 percent C and over 1 percent Mn from the filler metal and base metal.
Normal weld cooling transforms this composition to martensite at 30 to 35 Rockwell C hardness, possibly higher. This composition will have a low ductility and is susceptible to delayed cracking due to the action of diffusible hydrogen. Examine the weld at least a couple of days after welding to check.
Even if delayed cracking does not occur, this is still a brittle weld and any shock loading could result in a fracture. If the shop chooses this route, it needs to assess the potential consequences of a brittle fracture including personal injury, damage to equipment and potential litigation. Instead, I suggest the normal GMAW filler metal selection of ER309LSi.
Q: I am a bridge engineer working on a rehabilitation project. We are converting an old steel railroad truss into a bike path. As part of the bike-path support system, we will weld "new" steel to existing steel. The existing steel truss was built in the late 1800's. Are there any special precautions, filler metals or procedures for a field welding of this type?
A: Steel produced at the turn of the century (late 1800's — early 1900's) may have been made in a Bessemer process and may have a high level of sulfur or laminations made up of manganese sulfide inclusions. The first step is to obtain a chemical analysis of each of the old steel materials that will be welded to the new ones. The analysis should include: carbon, sulfur, mangenese, silcon, phosphorus, lead and tin.
If both the sulfur or phosphorus levels are lower than 0.050 percent, then the old steel is weldable with an E70XX low hydrogen electrode. Welding problems start to increase as the S and P levels go from 0.050 percent to 0.10 percent. The steel can not be welded with levels of S and P above 0.10 percent. The problem is hot cracking due to the low melting point of the tramp materials. Engineers in those days were not concerned with welding — they used rivets.
Besides checking the chemistry, follow good, low hydrogen welding practices such as pre-weld cleaning and preheat depending on material thickness and restraint. Finally, there is also a potential for laminations along the rolling direction of the steel. When loaded by welding shrinkage forces transverse to the rolling direction, these inclusions could open, resulting in failure. Contract a testing agency to inspect for laminations or inclusions using ultrasonic inspection in the welding areas.