November/December 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 | N O V E M B E R / D E C E M B E R 2 0 1 8 6 8 PROBLEM Variable valve timing (VVT) plates are part of a system designed to optimize engine performance by changing the lift, duration, and timing of valve lift events. An automotive parts supplier encountered frequent cracking of VVT plates that were heat treated at an outside source. The cracking problem resulted in high costs for part testing, material loss, and production time wasted on parts that had to be scrapped. The parts supplier approachedmetallurgical engi- neers at Paulo to review the problem to determine the cause of cracking and recommend a solution. BACKGROUND In their investigation, Paulo engineers found that the parts were brittle, and metallographic examination showed that the parts were carbonitrided by the heat treater, which embrittled the material. It was necessary to determine why the parts were carbonitrided. A review of the starting mate- rial showed that the parts were made from blanks stamped out of AISI 1045 carbon steel sheet in the annealed condition, purchased from a steel processor. Annealing was necessary to spheroidize the microstructure, which aids in fine blank- ing to close tolerances. Annealing was carried out in a nitro- CASE STUDY: ELIMINATING CRACKING OF VALVE TIMING PLATES gen atmosphere. This was a problem because the steel con- tained aluminum (used as a grain refiner in the steelmaking process), which combined with nitrogen in the atmosphere to form aluminum nitrides. These create a much finer grain size on the part surface (Fig. 1), which prevents full hard- ening of the material. Paulo suspected that the customer’s heat treater attempted to overcome this situation by carbo- nitriding. Although the parts hardened, they became brittle because 1045 steel lacks the hardenability required to over- come the fine grain size. Paulo purchased the same material from the steel processor and was able to recreate the brittle- ness by carbonitriding. SOLUTION Paulo asked the steel processor if thematerial could be annealed in a different atmosphere without nitrogen, which it was unable to do. The next best approach was to “spike” the 1045 steel with another alloying element that would in- crease hardenability despite the fine grain size resulting from the aluminum-nitrogen interaction during annealing. Chro- mium was pinpointed as the ideal element, and after some trial and error experiments, a formula was identified for the chromium spike that would result in fully hardened parts without cracks after through hardening (Fig. 2). Fig. 1 — Annealing AISI 1045 steel sheet in a nitrogen atmosphere to enhance fine blanking created a near-surface layer of finer grains (top). After carbonitriding by the manufacturer’s heat treater to help through hardening, cracking occurred. A chemistry modification recommended by Paulo (Cr addition) to the 1045 material eliminated the fine grain size during the steel processor’s annealing treatment (bottom) and solved the problem. 16