The Cheesy Casting

September 3, 2006 at 9:37 pm 12 comments

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).

housing wheel
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.

housing casting 01
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.

housing casting 02
Fig. 3. A near-surface porosity on the bearing housing.

housing casting 03
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.

housing solid
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.

housing-gantry
Fig. 6. Coal Transhipment System sits on 4 bogies (Inset: Slab-made bearing housing).

** End of Article **

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Entry filed under: Project Management, Steel Fabrication.

Structural Designers Take No Psychotropic The Tricky Ribs

12 Comments Add your own

  • 1. Mari  |  September 4, 2006 at 1:06 am

    Interesting πŸ˜€ There is so many twists and bends in completing a project yeah :O

    Reply
  • 2. isadikin  |  September 4, 2006 at 2:25 am

    That’s rite. Most of the problems show up in the field. When executing project, we must prepare for the worst. There are many compromises between time, money, quality, and safety. There are many on-site re-negotiations. Compared to project execution, design and calculation are the easiest and most peaceful stages in a project. πŸ˜€

    Reply
  • 3. Goio  |  September 4, 2006 at 8:14 am

    “… Casting product is inferior in strength and, for certain geometry, is more expensive compared to other fabrication process…”

    really? I thought that it would be cheaper and easier using casting product if we want an object with difficult geometry and size. I must have been wrong… afterall, I got C minus for my Manufacturing Process I AND class .. hihihihihi …

    Reply
  • 4. Goio  |  September 4, 2006 at 8:15 am

    sorry … I mean Manufacturing Process I AND II…

    Reply
  • 5. isadikin  |  September 4, 2006 at 11:08 am

    Goio: You are right, mate. If the geometry is difficult and complex, say: an engine block, casting will be cheaper because:
    1. It is difficult to find a solid steel the size of engine block,
    2. A lot of material is removed and wasted.
    3. Engine block has many complex round and hollow section which require heavy machining process. Remember: Process of removing a certain volume of material with machining (turning, milling, shaping, etc.) is more expensive than the removed material itself.

    The larger the size of component means the bigger the required machining center. Large machining center is very expensive and only big workshops have it. Because the equipment is expensive, workshop charges you higher rate to cover their return of investment when you use their large machining center.

    Finally, a tips for designer: Avoid the need to machine the part in large scale. For example: Avoid high-precision large diameter interface which needs large-size turning machine. Avoid long-distance mounting alignment which requires a large machining center with large working bed.

    In case of the bearing housing, I flame-cut the internal hole before send it to lathe to save the machining cost. Flame cut costs you peanuts and almost every workshop has flame cut. For the flat interfaces on the L-shape which must be 90 degree (precision), I used standard size shaper which requires easy and inexpensive angle alignment and setting. How can it be cheaper than casting? I didn’t pay attention to my Manufacturing Process class too, mate. πŸ˜€ But the fact is the whole thing is cheaper and faster than some quotations I got from steel casters in Jakarta. I think the simple logic behind that is:
    1. Casting looks deceivingly cheaper, but in fact, it requires more energy for melting the steel. And bear in mind that we also need additional energy to heat the container first. That’s why some steel casters ask you for a minimum total weight of cast product.
    2. It requires additional time and cost for preparing the casting mould.
    3. Casting is not a precision process due to the expansion-contraction during changing of temperature. We still need to machine the interfaces.

    Reply
  • 6. adi  |  September 11, 2006 at 2:46 pm

    In Hydraulic excavator there is many casting part, for example boom, arm and link. I think casting part is a must for joint area (like arm-boom) to reduce vibration effect

    Reply
  • 7. isadikin  |  September 13, 2006 at 2:56 am

    You are perfectly right, Di. Heavy-duty excavator usually uses cast steel for joint between frame-boom, boom-arm, and arm-bucket. Some manufacturer even use cast steel for the slewing bearing support on the undercarriage.
    I had this opportunity of observing old heavy-duty crawler crane, made circa 1970s. Most cranes and excavators from this era didn’t use slewing bearing. Instead, they use several (usually 4) steel rollers to clamp and roll around a turntable. Both the rollers and turntable are cast steel. And it is subjected to severe alternating load which is the source of fatigue failure. Those cast parts in excavator are of high quality ones. They are used in zone of high alternating stress.

    I am not very fond of using cast product in my design because it is hard to find good steel caster. You have seen what nightmare bad caster can give you in above article. We can ask them to provide quality assurance such as Ultra-sonic Testing or other NDT but they will charge you additional cost for it. And NDT is not cheap. Even if you are a rich project manager and willing to spend so much money to do NDT, still it is dificult to assess the quality because most NDT can only detect defect to a limited depth. Remember, most cast parts in heavy equipments come in thick cross section.

    If we can find a good steel caster, the price is very expensive and they only supply with minimum quantity (mass production). Steel caster always asks you about total weight of your cast part. If the quantity is too small, they will charge you extra cost or simply refuse your inquiry.
    Hydraulic ramps in excavator boom and arm are also supported by bushing integrated to cast housing. This is one thing which drew my attention when the Terex mining-class ecavator delivered to our project site. I spent few hours observing and taking photograph of the superstructures and mainframe when it was still in dismantled condition. I’ll write another article on this cast joint in excavator and try to find the reason behind cast steel process selection (one reason is as per your comment above: to reduce vibration). Actually, I took many pictures on the Terex cast joint. But my friend Mr. Jim Lees, Terex Business Manager for Indonesia, won’t be happy if I expose too much of his company’s products. πŸ˜€

    Reply
  • 8. leon  |  January 8, 2007 at 10:40 pm

    dear isadikin,
    what a great blog, having read all the articles i find your best article about casting; only little casting information is available. It would be most interesting to learn more about it, would it be possible you publish some info. about the Terex cast joints ?
    Anotehr disadvantage of casting with heavy equipment I learned was that the accuracy of the delivery is often less than one would expect.

    Reply
  • 9. isadikin  |  November 24, 2007 at 1:02 pm

    Hi Leon. Sorry for the long overdue reply. I’ll try to write more about cast joint in excavators. Casting products have poor accuracy since it shrinks during cooling down. But this is not a serious problem since you can always finish it with machining.

    Reply
  • 10. St Louis Malpractice  |  August 23, 2008 at 5:08 am

    Nice site….

    Wasn’t looking for this, but great stuff….

    Reply
  • 11. oguntoye  |  November 14, 2008 at 8:20 am

    well done GOOD JOB i wish to learn more

    Reply
  • 12. bruce  |  June 3, 2010 at 10:02 am

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    Our company is major in designing and producing metal parts in China

    For the company detailed and products information, please refer to our website: http://www.huahemetal.net.

    Your earliest reply would be highly appreciated and expected.

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    Reply

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