Case studies 441

18.9 Case studies 441

Table 18.12 Translation for joining a steel radiator and outcome (potential process-sensitive

defects and properties Function

• Domestic radiator

Objective

• Minimize cost

Constraints

• Material compatibility: low-

carbon steel • Joint geometry: lap joint • Mode of loading: tension,

shear (moderate)

Technical constraints

• Sheet thickness: 1–2 mm • Joint must conduct heat • Joint must be watertight • Service temperature ⬎100°C

Environmental constraint Free variable

• Disassembly not required

• Choice of shaping process and

process operating conditions

冎 service life

Defects

• Residual stress

Influencing fit-up and

• Imperfect joint

Table 18.13 Short-list of processes for joining a steel radiator

Brazing Electron beam welding Explosive welding Metal inert gas arc welding (MIG) Tungsten inert gas arc welding (TIG) Laser beam welding Manual metal arc welding (MMA) Oxyacetylene welding Riveting Soldering

change the short-list further. The processes passing the screening stage are listed in Table 18.13.

Quality and economic criteria are difficult to apply as screening steps in selecting joining processes. At this point we seek documentation for the processes. This reveals that explosive welding requires special facilities and per- mits (hardly a surprise). Electron beam and laser welding require expensive equipment, so use of a shared facility would be necessary to make them eco- nomic. Resistance spot welding is screened out because it failed the require- ment to be watertight. This is only necessary for the edge seams, so internal joint lines could be spot welded. This highlights the need for judgement in

442 Chapter 18 Heat, beat, stick and polish: manufacturing processes

Inner race

Outer race

Figure 18.25

A section through a ball bearing race. The surface of the race is to be hardened to resist wear and fatigue crack initiation.

applying automated screening steps. Always ask: has an obvious (or existing) solution been eliminated? Why was this? Has something narrowly failed, so that a low-cost option could be accommodated with modest redesign? Could a second process step, like sealing the spot-welded joint with a mastic, correct a deficiency? The best way forward is to turn constraints on and off, and adjust numerical limits, exploring the options that appear.

Surface hardening a ball-bearing race

The balls of ball bearing races run in grooved tracks (Figure 18.25). As discussed in Chapter 11, the life of a ball race is limited by wear and by fatigue. Both are limited by using hard materials. Hard materials, however, are not tough, incur- ring the risk that shock loading or mishandling might cause the race to fracture. The solution is to use an alloy steel, which has excellent bulk properties, and to apply a separate surface treatment to increase the hardness where it matters. We therefore seek processes to surface harden alloy steels for wear and fatigue resistance. The precision of both balls and race is critical, so the process must not compromise the dimensions or the surface smoothness. Table 18.14 sum- marizes the translation.

The CES system contains records for 44 surface treatment processes. Many are compatible with alloy steels. More discriminating is the purpose of the treatment—to impart fatigue and wear resistance—reducing the list to eight. Imposing the requirement for a very smooth surface knocks out those that coat or deform the surface because these compromise the finish. The short-list of processes that survive the screening is given in Table 18.15. Adding the further constraint that the curved surface coverage must be very good leaves just the first two: carburizing and carbonitriding, and nitriding.

To get further we turn to documentation for these processes. The hardness of the surface and the depth of the hardened layer depend on process variables: the time and temperature of the treatment and the composition of the steel. And economics, of course, enters. Ball races are made in enormous batches and while their sizes vary, their geometry does not. This is where dedicated equipment, even if very expensive, is viable.