This paper is adapted from article published by BMR Group in Wire Technology International, November/December 1995, page 67
Laser hardening can reduce capstan surface wear for improved wire quality. That improves your production efficiency and your bottom line.
No matter how much money you spend on highly capable wire drawing equipment, capstans always seem to present problems. Capstans surfaces take a lot of abuse from the wire, and if they don’t stand up to that abuse, your product quality suffers. Typical problems include grooving from wear, wire scuffing from soft spots, and rough finishes from improper substrate material. When these problems occur, the capstan has to be removed from operation and resurfaced, and downtime results.
LASER HARDENING TO THE RESCUE
A specialty wire fabricator in Oregon was awarded a development contract to draw superconductor wire for a high-visibility project involving government and industry resources. However, shortly after they started operations, they began to experience serious problems with the wire contact surface on one of their fabricated steel capstans, which was 54″ diameter with an 11″ face with 1-degree taper. The face material, which was 1045 steel, was originally flame hardened, but it developed some soft spots that scuffed and marked the wire. Within just a few weeks of service, they began to generate an unacceptably high level of scrap. As a result they began to fall significantly behind schedule on the project.
Fortunately, this fabricator found a solution to its surface wear problem at Light Beam Technology, Inc. (LBT), based in Wolf Lake, Indiana. LBT reground the capstan surface to remove the wear pattern, then used its 5KW CO2 laser to case harden the contact surface. LBT completed the process by polishing the surface to the specified finish. The capstan was then returned to service and the fabricator experienced no further problems for the duration of the project.
WHY LASER HARDENING?
According to LBT president Larry Boyd, laser hardening the contact surface is sometimes the only solution for large steel capstans with wear problems. The typical method used to produce a hard contact surface is welding buildup, but this causes high dilution of the surface material that results in only fair wear resistance and a surface that is highly susceptible to cracking. Thermal sprayed coatings work well for capstans used with smaller wire, but these coatings do not hold up well to high stress loadings imposed by larger wire.
Flame hardening, which is sometimes used to case harden contact surfaces, poses a great risk of distorting the part, and it can leave soft spots. Boyd claims that laser hardening produces a uniform, hard surface with no risk of distorting the capstan. That’s because the laser beam used for the heat treating process can be focused on an extremely small area—as little as 1/4″ square. As a result only the desired surface area of the capstan is heated, leaving the remainder of the part unaffected. And case hardening the surface area by laser avoids the surface dilution and cracking problems found with standard welding processes.
Boyd emphasizes that laser heat treating is not the answer for all capstans. “The material in the surface area has to be hardenable,” he says. “That means it has to have at least 0.4% carbon or heat treating isn’t an option—by laser or by any other method.” He claims that the laser typically produces a case hardened surface of 58 to 62 Rockwell C on materials like 1050, 4150, or A2 steel. This provides an exceptionally hard surface that can withstand the stress of continual wire contact without compromising the ductile strength of the substrate material.
According to Boyd, laser heat treating provides a different result from other heat treating methods in several respects. First, because the beam can be controlled with such precision, laser heat treating can be applied to a small section of the surface. This might be needed on capstans with very narrow contact surfaces. However, the entire wire contact surface of a very large capstan can also be hardened by repeated passes through the laser beam on one of the company’s two CNC workstations.
Second, the heat input from the laser beam is concentrated in a very small area of the part. This creates extremely rapid heat buildup and allows extremely rapid cooling, leaving a hardened surface of 0.020″ to 0.060″ depth. Boyd notes that penetration greater than 0.080″ is usually not advisable with the laser because of the risk that the surface area will start to melt. This would defeat the purpose of the heat treating process and result in surface distortion.
Third, the depth of penetration can be controlled within 0.010″ to 0.020″, which means very thin or non-uniform parts can be treated without distortion. Control of the depth is critical because if the hardened surface is too deep, the entire assembly can become brittle and crack. The actual penetration depths achieved with laser heat treating correlate very highly with predicted depths.
Boyd is quick to point out that Light Beam Technology also offers a wide range of other processes for extending surface life on smaller non-hardenable capstans. “We also do a lot of sprayed coatings of everything from carbide to ceramic,” he says. “Many times we won’t know what kind of process the customer needs until we get the part in-house. Before we can start to solve the problem, we have to determine if the substrate material is weldable, the amount of buildup it will need, and what kinds of treatments it may have had in the past.”
For a typical laser hardening application of a capstan, LBT grinds the surface OD true, hardens the surface, then polishes it to the required smoothness. Boyd says the company has the capability to produce a superfinish as fine as 4 Ra.
One of the other processes Light Beam Technology offers to combat contact surface wear is laser cladding. In this process, a powdered metal is applied and bonded into the surface material by the laser. Although laser cladding produces a small amount of surface dilution, the resulting clad surface has excellent wear resistance and is only minimally susceptible to cracking. Another advantage of laser cladding is that the built-up surface, which can be up to 0.120″ in thickness, can be tailored to any desired material composition.
Boyd says he recommends the laser only when other approaches won’t meet the customer’s requirements. “We try to keep the laser dedicated to appropriate jobs. So, if a different process can solve the customer’s capstan wear problem, we’ll usually recommend that process. But sometimes, if they have a line down, they want the capstan back as soon as possible. If there is no major surface wear, laser hardening is the fastest and best way to get it back in service.”
Other Wire Applications for Laser Hardening
The benefits of laser heat treating for wire manufacturers isn’t confined to large-diameter steel capstans. Many other fabricated and cast components used in the production of wire, such as coiler wheels, bearing and bearing seat areas, splines, tapered arbors, and machine rails can also benefit from this technology.
Any components that are subject to wear, abrasion, or galling are commonly hardened by laser. The special value of the laser is that it can harden selective areas of the component without affecting the rest of the part.