Groover: Fundamentals of Modern Manufacturing, 5e

Case Study by Daniel Waldorf, California Polytechnic State University

Heat Treatment Case Study: Defective Bolts for Hatchback Wagon

Damian works as a quality control engineer at the manufacturing plant in Tennessee that assembles the new XR4 hatchback wagon. He is responsible for preventing and correcting any quality problems related to the vehicle assembly process. In recent months, very few quality problems had materialized, and so Damian has been able to get started on a couple of prevention projects that have been on the back burner. Suddenly, however, his phone rings, and his attention is turned towards a developing crisis.

Four 6-mm diameter bolts are used to hold on the rear hatchback door. The bolts are made of hot rolled 0.8% plain carbon steel that goes through a heat treatment prior to use in assembly. The phone call was from the heat treating supplier. In the most recent shipment of bolts, three of the four sub-lots of parts were treated as usual (i.e., heat to 1440ºF for 30 minutes, water quench, and then a 1.5-hour temper at 800ºF), but the fourth sub-lot had a problem. Due to a personnel mix-up combined with a system monitor malfunction, the supplier can not tell exactly how long the parts had been tempered. It is possible they were processed properly, but just as likely that they were not tempered at all, making them too brittle and likely to crack during use. Unfortunately, the questionable sub-lot was unknowingly mixed with the good lots just before shipment. The fourth sub-lot was quite a bit smaller than the others, so the supplier estimates that about 1 in 30 bolts may be suspect. He offers to rework he parts, but says it could not be done in less than two days.

The shipment of mixed-up bolts arrives that day at 11am. The bolts are needed for assembly right away or the plant will start losing money from idle workers and equipment. Damian could have the bolts tested, but hardness testing is time consuming and they may have to test dozens or more bolts before finding a defective, if they find any at all. On the other hand, because of the redundancy in design, Damian knows that if just one defective bolt ends up on a vehicle, not much trouble would likely come. But if two or more bad bolts ended up on the same car, the chance of a real problem is much greater. Damian uses his probability textbook from college to compute the probability of each number of defectives out of a random sample of 4 if 1 out of 30 bolts are defective:

X = 0 1 2 3 4

Prob of X Def. out of 4 = .8732 .1204 .0062 .0001 ≈.0000

The production supervisor, the assembly operator, the quality inspector, and the heat treatment supplier all encourage Damian to recommend just using the bolts as is, since the likelihood of a real problem is small. Production pressures are made worse because it is near month’s end and product is due for immediate delivery. Damian cannot locate his boss and is unsure how to proceed . . .

WATCH THE VIDEO: Iron-Carbon Phase Diagram

1.  What happens to steel if it is held above 1670ºF?

2.  What is the maximum amount of carbon that can be dissolved in ferrite at any temperature?

3.  What happens to the excess carbon in a steel if it is slowly cooled to ferrite while containing more than the maximum amount dissolvable in ferrite?

4.  What range of carbon percentages covers most steels?

GO TO THE TEXT: Chapter 26

5.  Martensite is an alternative microstructural form of steel that is both hard and brittle. What causes the extreme hardness? See Section 26.2.

6.  What is the purpose of the tempering step in the case study? Is it really necessary for this application? See Section 26.2.

7.  If the bolts in the case study had been carburized rather than the heat treatment described, how would they be different? See Section 26.4.

8.  What is the probability of 2 or more defective bolts ending up on the same vehicle based on Damian’s computations? What should Damian do about the situation?

SOLVE

9.  About how hard are the parts in the case study before heat treatment? See the value in Table 3.6 for high-carbon steel, and compare to the valued for annealed plain carbon steels in Figure 26.2.

10.  If the water quenching process in the case study cools the metal at a constant rate, use Figure 26.1 to estimate the maximum length of time it should take to cool the parts to room temperature. Assume full martensite formation.

11.  Approximate the parts’ hardness after quenching but before tempering? See Figure 26.2.