Heat Treating Wedges
In October 2023, I competed at Utah Combat Robotics’ Fall Brawl. I was trying out new wedges for Piano made from 4130 Steel rather than Grade 5 Titanium. The 4130 wedges are far more cost effective than the titanium wedges. I was also able to make the wedges wrap around to the side of the robot without the need for hot bending. However, the 4130 was slightly soft for combat. The prototype wedges received cuts from saws and chunks removed by horizontal spinners.
To mitigate this, I heat treated a new set of wedges to harden them. I referenced a Time-Temperature-Transformation (TTT) Diagram to determine what temperatures the steel would need to be hardened. To ensure the steel was fully Austenized prior quenching, the temperature was set 50° above 1550° F, the Austenizing temperature. The wedges were allowed to soak in the oven for 1 hours before being water quenched.
I used the wedges on Piano at December 2023 ARC. Piano went 0-2 against very difficult opponents, one of which went on to win the event. The wedges took bends that were more difficult to remove, due to the lower ductility. However, with some geometry improvements, I decided, the hardened wedges were more likely to stand up to combat than the unhardened wedges.
Moving forward with keeping the wedges hardened, I removed the slots in the wedges for fork mounting and removed the thin geometry. I ran these wedges at multiple competitions in 2024. They performed amazingly against many scary robots, absorbing massive hits and dissipating energy into the wedge mounts safely. But, towards the middle of the year, I started having issues with the wedges cracking along the bends when hit especially hard by massive weapons.
I had two theories for why the wedges started cracking: internal stresses and high hardness. To mitigate these factors, I started annealing the wedges before heat treating and tempering after. The annealing reduces the internal stresses caused by the bends and the tempering allows me to lower the hardness of the wedges post hardening.
To determine the desired hardness for my wedges, I looked at typical weapon hardnesses. Many robots use laser cut AR500 weapons. This steel should be Brinell 500 hardness, but the heat from laser cutting alters the surrounding crystal structure and produces a “Heat Affected Zone” (HAZ). This HAZ has lower hardness than the rest of the steel. I estimated the hardness of the HAZ to be approximately Brinell 400-450 (approx. 43-48 HRC). From discussions I had with other combat robot builders, the hardness of the laser cut AR500 may be even lower than 400. From that information, I decided tempering the wedges to 45 HRC would provide a balance between hardness and toughness.
These new wedges are the most durable I have competed with. They have absorbed many high energy kits from opponents. I loved the process of making them and I think they will last for a long time. For your entertainment, I’ve included a video of my fight with Sandstorm at OC Maker Faire 2024.
