TIPS FOR OVERLAYING JOURNALS AND CASTER ROLLS

Although the ultimate hardfacing deposit selected will vary based on the intended application, follow these welding and fabrication techniques for rebuilding caster rolls, crankshafts, roll pins, extrusion rolls, crusher rolls, or other types of journals made of low alloy steel.

Over the past few years, I can safely say that I have fielded questions regarding roll rebuilding on an average of  about one-per-month. So my hope is that the readers of this month’s column may find the tips contained within useful.

Quite often these questions would be in regard to caster rolls, as mills can utilize hundreds of these on a single line to draw billets of steel. However, the questions I receive are just as often in regards to crankshafts, roll pins, extrusion rolls, crusher rolls, or other types of journals – most of which were made of low alloy steel. Please note here that “low alloy” refers to steel with significant additions of chromium, nickel, molybdenum and other elemental additives in addition to higher levels of carbon.

Some of the rolls have base metal grades including 4130, 8620, and SA649. Although the ultimate hardfacing deposit selected will vary based on the intended application, the welding and fabrication techniques will be similar.

Because many of these rolls are subjected to thermal shock and highly abrasive environments, roll wear can be exceedingly rapid and in their original state, their effective life can be quite short. Rather than allowing wear and tear to turn these high value components into scrap, the decision makers at many profitable businesses have chosen to rebuild these rolls prior to the end of their useful life.

The layers of overlay deposits used to accomplish this rebuilding process are devised to handle the extremely high temperature (possibly 1400 deg F or higher), high pressure, corrosive and metallic oxide abrading environments to which the rolls are subjected. I have used the term “devised” because – generally speaking – successful roll overlay is most often achieved through the use of a few different layers of deposits, and not solely one layer of high-hardness weld metal.

Typically, this process consists of a butter pass (about 1/8 in to ¼ in thick), a build-up layer (about ½ in thick), and a final, high-hardness finishing layer appropriate for the situation. A butter pass of mild steel weld metal – which lessens the chance for spalling – is highly recommended when the carbon content of the base material is 0.30 percent or higher

An example of a mild steel weld metal deposit would be Lincolnweld^® L-61 submerged arc solid welding wire and 801 Flux. Alternatively, a gas-shielded welding process such as flux-core arc welding (FCAW) with UltraCore^® 71C or gas metal arc welding (GMAW) with SuperArc^® L-56 may be used. However, as many fabricators know, preheat and interpass temperature control will almost always be necessary due to the high carbon content.

Generally speaking, the preheat soak temperature used should be at least 10 deg F for every point of carbon (to illustrate, a 0.30 percent carbon steel is considered to have 30 points of carbon). Immediately following the butter layer, a build-up layer that closely matches the chemistry of the base material is typically warranted.

For example, Lincore^® 8620 submerged arc tubular wire with 801 Flux most closely matches the chemistry of a caster roll comprised of 8620 base material. As stated above, this build-up layer should be on the order of ½ in or more in thickness. The idea behind a build-up layer is that it should be thick enough so that the heat-affected zone of the outermost high-hardness weld layers will not extend into the base material. If this guideline is followed, the chances for cracking are further reduced. Otherwise, a post-weld heat treatment may be required to relieve internal stress within the roll.

And finally, at least one “outer skin” layer of overlay deposit comprised of high strength weld metal is deposited. These top layer deposits are explicitly designed to handle high temperature, highly abrasive and/or corrosive applications. These deposits can be comprised of high alloy steel (Lincore^® 102HC / 802 Flux combination with greater than 6 percent chromium content) or even martensitic stainless steel (for example, Lincore^® 410 or Lincore^® 420 with 801 Flux with their 13 percent chromium content). As one might expect, because these higher strength deposits are highly hardenable, they are extremely susceptible to cracking if proper precautions are not taken.Minimum preheat and interpass temperatures upwards of 600 deg F or higher are often required in order to push the deposit above the martensite start temperature. This allows for a fully martensitic microstructure to be established, which maximizes hardness and abrasion resistance. An additional benefit of this level of preheat and interpass temperature control is that it assists in reducing the cooling rate, thereby mitigating the chances for cracking in the top layer.

However, enhanced crack sensitivity can occur as a result of the way in which caster rolls can act as a heat sink (especially rolls that are solid versus “hollow”). To address this, in some situations it will be absolutely critical that the roll be wrapped in a fiberglass blanket to slow the cooling rate even further.

Even if all of these precautions are successful at reducing crack susceptibility, a post-weld heat treatment (PWHT) soak at a holding temperature of at least 850 deg F (for one hour per inch of thickness) may still yet be required. While at first this may seem counter-intuitive to maximizing hardness, this may be necessary because martensitic stainless overlay deposits offer the most optimum mechanical properties in the tempered condition.

However, the PWHT should absolutely not begin until the weld has cooled down completely. By allowing a cool-down after welding, a martensitic microstructure will be permitted to fully develop in the weld. Then, the PWHT will produce “tempered martensite”, which is considered to be an ideal combination of strength and toughness for caster rolls.

Now that we have the “pre-game” analysis of the base material, weld layer types, and temperature control squared-away, let us now tackle the actual welding considerations for roll re-building. Based on years of collective welding experience at my company, a rule-of-thumb for developing starting procedures has been developed for 1G position weld overlay applications. This tried-and-true heuristic is that a starting point for the deposition rate (in pounds per inches) is equal to the diameter of the roll (in inches) being hardfaced.

For example, one would expect to be able to comfortably deposit weld metal at a rate of 12 lb per hour on a 12 in diameter roll. Most of the time, the deposition rate gleaned from this ratio will be below the maximum amount of weld metal that can be balanced on the top of the roll before gravity takes over and causes weld metal to roll out of the puddle.

Please keep in mind, however, that this 1-to-1 relationship between deposition rate and roll diameter is only a starting point. I have personally been involved in projects where our application teams have been able to increase the deposition rate to levels that are considerably higher than what this rule-of-thumb would predict.