The Cheesy Casting
Designer should be aware of how his design will be fabricated. Alongside doing design, I am also in charge of the whole construction process and procurement. My last experience with casting product was a bad one. Casting product is inferior in strength and, for certain geometry, is more expensive compared to other fabrication process. Of course there are many good casting product but they are usually more expensive and take additional process and treatment (forging, machining, case-hardening, etc.). Take a look at the following assembly in Figure 1: Wheel set (Wheel, shaft, bearing, and bearing housing).
Fig. 1. Wheel set: Wheel, shaft, bearing, and bearing housing.
A designer can simply draw a wheel set and assign certain required properties such as load capacity. But for the construction engineer, working out the design to reality is much more complicated. Take the wheel for example. This wheel is made of a series of high-quality process. Casting, forging, machining, and case hardening. We imported this wheel from the States and it is very expensive.
Casting Defect: Porosity
The round L-shape steel block is bearing housing. Each of this housing is designed to take 20 ton load. It is a critical part since it is the medium of load transfer from wheel to frame. Due to geometry, the most logical and cost effective process for this housing is casting. We awarded the contract to fabricate this housing to a local company. We were surprised when we saw the delivered product (Figure 2). The surface is rough and full of pores. The sub-contractor painted them before they sent them to cover the defects. We blasted all the painting away and inspected the housing more closely. The result was a nightmare.
Fig. 2. Casted housing with painting partly blasted off.
We found many cracks and porosities. Porosity is the result of poor casting process in which gas is trapped inside the casting. A good caster will properly design mould with adequate ventilation to allow gas release and avoid porosity. Figure 3 shows a sample of serious porosity in one of the housing. We found a suspicious spot. When we hit that spot, it simply broke away (notice the scattered material around the pointed hole) and exposed a deeper porosity. Finally, we cut out a piece of the housing around the porosity and it revealed a nightmare shown in Figure 4. The housing turned out to have more holes than a swiss cheese.
Fig. 3. A near-surface porosity on the bearing housing.
Fig. 4. Cut-out section of the housing showing larger porosity.
What should we do in this situation? We were then at critical point of project in which right decision must be taken as fast as possible considering the time limit, extra cost and quality of structure. We condemned all the cast product. To go back to the sub-contractor and ask for better quality product is out of question. First, how to make sure they can do better than what they gave before? Second, we ran out of time for another casting. Steel casting takes longer delivery time because of additional mould preparation time. Looking for new and better caster will take even longer.
Fabrication Alternative: Steel Slab
I took over the housing procurement and made unusual decision: Cutting and machining the housing out of solid steel. Why unusual? Because the thickness of this housing is 140 mm and the thickest steel plate available in market is only 75 mm (and it is rare). How? Using steel slab. Steel slab is a semi-finished product in steel mill for further production into sheet metal by hot rolling or cold rolling. The problem is: Steel slab is not for sale unless you have a steel mill and you order some hundred tons of slab for rolling it into sheet metal.
Here is the secret. Steel slab is a customized product. Usually steel mill manufactures a spare quantity of slabs to replace rejected item. Sometimes this steel slab is slightly bent due to improper handling during transportation in vessel, trailer or storing in warehouse. It is rejected and steel mill sells it to scrap yard. Some high-quality scrap yards and steel plate retailers sell this slab but with random thickness and limited availability. I managed to find a piece of slab with thickness +/- 140 mm. Cut it using oxy-acetylene, machined the primary interfaces with lathe, shaper and drill. After flame-cut, there was about 5 mm layer of hard layer on the cut surface. This is due to the effect of case hardening from the flame. Thus, be careful when you try to machine off this layer. The surface can be as hard as the machine tool tip. Adjust the feeding rate and speed properly. Use low feeding rate.
Fig. 5. Cutting steel slab to form bearing housing.
This rusty thing was from scrap yard. How about the quality assurance? No problem. We know steel only corrodes on the surface. The internal part remains intact.
But steel slab is not as strong as rolled sheet metal? No problem. it is still stronger and superior to a cast steel anyway.
How about mill certificate? No mill certificate. It came from a scrap yard. I took a piece of the cut slab to the laboratory and gave it few shots of Mass Spectrometer. Spectrometer gives the major chemical compositions and that serves as the certificate. Based on Spectrometer reading, we can classify the steel into nearest steel grade.
How to find the steel slab of exact dimension in scrap yard? That is the art of project execution :D.
Where is the slab-made bearing housing today? It is somewhere in Kalimantan river, carrying a 300-ton moving structures on top of a 300-feet barge, shown in Figure 6. What will happen if we use the swiss-cheese cast housing? God only knows. Finally, we had the housing delivered on schedule. It is cheaper and much stronger than casting.
Fig. 6. Coal Transhipment System sits on 4 bogies (Inset: Slab-made bearing housing).
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