Steel Selection and Heat Treatment

Quick Summary

There is a trade-off between hardness and toughness. Harder dactyls stay sharp but are more prone to snapping. Most wood-cutting tools have a hardness of about 50 Rockwell. This is hard enough to hold an edge while cutting wood without sacrificing too much toughness.

Good steel selections for ~50 Rockwell hardness are steels with 0.3% to 0.4% carbon content (4140, 4130, etc.) These steels are fairly easy to harden and will provide the desired toughness.

One way to achieve high hardness without sacrificing toughness is to case harden the part (this is often done on gears). Essentially only the outer layer is hardened, making it resistant to surface wear without making the entire part brittle. Details on case-hardening methods may be found below.

There are places parts can be sent to for heat treating. Many offer their services free to students/non-commercial endeavors.

A handy PDF outlining various steels and heat-treatment processes found on the McMaster Carr website may be found on this page.

A Bit More Detailed

General Heat Treating

Heat treatable steels have carbon content ranging from 0.06% to 2% Higher carbon contents result in harder, more brittle steels. Steels with high carbon content are often referred to as 'tool steels' since cutting tools are often made out of them. The W1 and D2 dactyls are made out of tool steels with carbon contents around 1.5%.

The hardening of steel is achieved through a phase transformation. Steel is heated until it forms austentite then rapidly cooled. This locks the steel into the hard but brittle martensite form. After quenching, the part is heated again to a much lower temperature in a process known as tempering.

Austentizing

The temperature at which a particular steel alloy will form austentite can be seen on the phase diagram below. Austentizing is often done only slightly above the temperature at which austentite begins to form for the particular alloy. Once the part reaches the correct temperature, it's best to leave until it reaches equilibrium. The time required for this depends on the thickness of the part. I've seen time recommendations range from an hour to ten minutes for small parts.

Quenching

There are a wide varieties of quenches available. Different quenches provide different cooling rates, water is the fastest, followed by oil then air. Faster quenches are more likely to form martensite, however, cracks might form in the part if the outside cools too fast compared to the inside. Slower quenches are less prone to cracking, but also might not result in the formation of martensite. The W1 dactyls were water quenched and the D2 were air quenched.

Tempering

Tempering temperatures depend on the particular alloy of steel used and how hard/tough the part needs to be. Tempering temperatures range from 200-600˚C. Tempering slightly reduces the hardness of the part, but greatly increases it's toughness. The higher the tempering temperature, the tougher the part will be, but with less hardness. The W1 and D2 dactyls were tempered at about 400˚C.

Case Hardening

As was mentioned earlier, case hardening only hardens the outside of a part, leaving the inside untreated.

Flame Hardening

Flame hardening uses a gas flame as a heat source to austentize the outer layer of material. This process is most commonly done on low carbon steels (0.4% to 0.95% carbon) with 0.45% carbon being the most popular. Flame hardening can be done with higher-carbon steels, but they are prone to cracking. Flame hardened parts should be tempered afterwards.

Induction Hardening

Induction hardening is essentially the same as flame hardening except induction is used as the heat source.

Carburizing

Carburizing starts with a low-carbon steel and uses diffusion to increase the carbon content of the outermost layer. When this part is heat treated, the outer layer becomes harder than the inner core, resulting in a case-hardened part.

To carburize a part, start with a low-carbon steel (about 0.2% carbon) and pack it in a sealed steel container with activated charcoal. Bake this at about 850˚-950˚C for about 2 hours. This will create a ~1mm thick layer of 1% carbon steel all around the part. Let the container cool, then heat-treat the part at 900˚-1000˚C, quench it in water, then temper.

Other Methods

Other methods of case hardening exist (e.g. gas nitride), but seem a bit too complicated to try in-house. Professional heat treatment shops have the equipment and experience to use such methods.

-- LaurelFullerton - 23 May 2008; minor addition MarkCutkosky

Preliminary Experimentation

Method

Initially, two types of dactyls were made. One set was made out of D2 tool steel and the other was made out of W1 tool steel. Some of the dactyls were heat treated in house with a propane torch. During heat treatment, the temperature of the part was determined by the color of the steel. The heat-treatment processes are outlined below:

D2 heat treatment process:
Austentize: Heat to 1000˚C (orange-yellow to dark yellow)
Quench: Air
Temper: 400˚C (dark grey oxidation) followed by air cooling (Done twice)

W1 heat treatment process:
Austentize: Heat to 800˚C (red to bright red)
Quench: Water
Temper: 400˚C (dark grey oxidation) followed by air cooling

Results

When tested on the RiSE robot, the W1 dactyls performed much better than the D2 dactyls. This result, however, was likely due the shape of the W1 dactyls rather than their material properties.

After a few test climbs, the tip on one of the untreated D2 dactyls bent backwards. No deformation, however, was noticed on the heat treated dactyls. This indicates that even though the conditions of the heat treatment were less than ideal, heat treating the dactyls can improve their performance.

Heat Treatment Companies in the Bay Area

Pacific Heat Treating Company
1238 Birchwood Drive
Sunnyvale, CA 94089
(408) 736-8500

Edwards Heat Treating Service
642 McCormick St
San Leandro, Ca. 94577
(510) 638-4140

Thermo-Fusion
2342, American Avenue
Hayward, CA 94545
(510) 782-7755

-- LaurelFullerton - 30 May 2008