May/June_AMP_Digital

FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | M A Y / J U N E 2 0 1 8 4 4 CASE STUDY: CORRECTLY SIZING AN INDUSTRIAL CONVEYOR OVEN In conveyor oven applications, product testing is critical to determine the part heatup time and therefore the oven length. Mike Grande Wisconsin Oven Corp., East Troy, Wis. W hen selecting or sizing an industrial oven, the time it takes to heat the load is an important part of the decision-making process. For batch applications where the product is placed into the oven and heated, the heating time determines whether the oven will have suffi- cient capacity or whether multiple ovens are necessary. In a continuous oven where parts are placed on a constant- ly moving oven conveyor, the heating time determines the distance the conveyor must travel in the heat, and therefore the length of the oven chamber. This article explores a case study of how product testing was performed for a hydrogen embrittlement relief process of fasteners to determine oven design parameters. HYDROGEN EMBRITTLEMENT Hydrogen embrittlement is a reduction in steel ductility caused by hydrogen contamination. It results in an increase in brittleness and decrease in yield strength. Of importance to manufacturers of fasteners is hydrogen embrittlement of high strength steel resulting from electrochemical surface treatments. It is caused by hydrogen from the process invad- ing the metal lattice and weakening the grain structure over time. If this condition is not addressed by heat treatment or other means, hydrogen embrittlement can lead to cata- strophic material failure. The most common method of preventing hydrogen embrittlement is through a heating process referred to as hy- drogen embrittlement relief, more simply known as baking. Baking consists of heating fasteners to a temperature be- tween 350° and 430°F and holding them at that temperature for a period of 8 to 24 hours (Fig. 1). The process takes place in a forced air convection oven, either a batch or conveyor style unit. The required heating time and temperature are determined by the hardness of the steel and the permeabil- ity of the coating. For example, for zinc electroplated fasten- ers above 39 HRC, common practice is a baking time of at least 12 hours. LUG NUTS CASE STUDY A manufacturer of automotive lug nuts was experienc- ing failure of its high carbon steel lug nuts (Fig. 2). After con- sultation with a metallurgist, it was determined they were dealing with hydrogen embrittlement caused by the electro- plating process. Hydrogen from the chrome plating was mi- grating into the steel, causing embrittlement. It was decided that baking would be incorporated into the manufacturing process to prevent this. The composition of the steel and the plating dictated that parts must be heated to a temperature of between 395°and 410°F and soaked at that temperature for 8 hours. To prevent hydrogen embrittlement, the baking pro- cess must occur immediately after plating, before the hydro- Fig. 1 — Baking time necessary for embrittlement relief. Fig. 2 — Automotive lug nuts are often chrome plated for corro- sion resistance and improved appearance. 12