Part 1:
When farm tractors are used on sticky or wet soils, it is common practice to attach dual driving wheels to the rear axles using fittings like that shown in the sketch. SAE grade 5 U-bolts are used to fasten the dual wheels to the axles and are expected to have "infinite" life. However several U-bolts made of 1 1/8 in (28.6 m) diameter rod broke in less than 100 hours of service.
The material for the U-bolts was commercial quality, cold finished AISI 1045 steel bar, 1 1/8 in (28.6 m) in diameter. Bar stock was cut to length, threads were cut on both ends, and the parts were bent to the specified shape. The U-bolts were then austenitized, oil quenched, and tempered to a hardness of Rockwell C 30 to 35 at the surface to obtain a uniform tempered-martensite microstructure. Oil quenching, rather than water quenching, was employed to avoid cracking of the bolts through the cut threads.
The design and processing for these failed U-bolts was based on a similar use in which 3/4 in (19 mm) and 7/8 in (22.2 m) in diameter U-bolts made of AISI 1045 steel and processed by the same procedure were in service on smaller tractors for several years with no failures.
Investigation which has been conducted shows the bolt legs in which failures occurred were all in the same position relative to the direction of wheel rotation. Visual examination showed that the break was a relatively flat transverse fracture in the threaded section between the washer and the nut. The appearance of the fracture surfaces was characteristic of failure by low-cycle fatigue with a smooth matte region showing beach marks, a region which generally extended over about 40 to 60% . of the fracture surface. The point of initiation of fatigue was at the root of the last thread at the edge of the nut on the side toward the washer.
A hardness traverse across a diameter of a failed leg indicated the hardness of the surface, to a depth of about 1/4 in (6.3 m), was in the specified range of Rockwell C 30 to 35. Interior hardness, around the center of the cross-section, was about Rockwell C 25.
Obviously, we would like to be able to advise our client on a solution to the problem. We shall look forward to hearing from you in the near future.
Thank you for your prompt response to my recent letter regarding failures in U-bolts for tractor axles. Your recommendations are appreciated. We have advised our client who will, I assume, act on them.
Our client has raised another question. He is aware that progress is rarely impeded by a lack of new knowledge but is much more likely to be impeded by failure to apply knowledge which is already readily available. He recognizes that his company has made these U-bolts in this manner for many years with reasonable profit. He believes this is a reasonable procedure with which he can continue to operate. In fact, he thinks this may still be the best procedure for his company. He also knows that the procedure has not been reviewed for several years and now there may well be other procedures which will give as good, or better, product at the same, or lower cost.
He has asked us to go back to "square one" and review the whole situation to see if there is a better procedure. As you know, we lack the necessary metallurgical expertise and are requesting your assistance in this matter. I shall look forward to hearing from you in the near future.
[Based on ASM International Metals Handbook, Vol 10, 8th Edition, pp 475 and private communication from Bruce P. Bardes.]
1. Some years ago we sought your advice on design and production of SCUBA tanks. You recommended using 7075-T6 alloy. We followed your recommendations and have been making and selling tanks which apparently have performed quite successfully as we have had essentially no complaints.
Recently, our marketing department has raised a question. That group has suggested that a painted decorative design applied to the tank might have significant customer appeal. Apparently, this even includes the possibility of marketing tanks in various colors to match swim suits, etc.
They believe a particular type of ceramic paint would be appropriate. There is good evidence that it will indeed adhere to an aluminum alloy base for a very long period of time and will resist a substantial amount of rough handling. We, in engineering, think the paint would be satisfactory in performance on tanks in customer use.
Application of this paint requires a 15 minute cure at 9250F (5000C). Are there any potential adverse consequences of using such a paint? If so, what are they? Why would such consequences arise? What steps could we take to avoid the consequences? Or, are we needlessly concerned?
2. A chain is used to hoist heavy loads. A link failed. Examination of this link showed substantial deformation and necking prior to failure by separation. What type of failure is this? Who is responsible? How can a similar failure be avoided in the future?
3. We have been approached with regard to supplying tools for a large Project, tools which are not in our current line. We are thus in need of information and recommendations which we r-an use to provide a basis for further consideration of the advisability of our entering into a new product area.
Specifically, we are interested in tools which are strong, yet non- sparking. Obvious uses for such tools are in oil fields, flour mills, etc., where highly flammable materials can be ignited by sparks.
In general, we are interested in producing wrenches, chisels, hammers, and similar hand tools. These must have a minimum strength of 1035 MPa. Obviously, high strengths, in the order of 1380 MPa, would be desirable. But the higher strengths are desirable only if they can be obtained without significant sacrifice of other characteristics, e.g., ductility.
Please advise us as to possible alloys with details of processing. We assume financial arrangements, similar to those in our previous dealings, will be agreeable to you.
4. We have been asked to design a component which can cope with all of the following conditions: (1) every 20 mS almost instantaneous pressure of 6.9 Mpa (1OOO psi), (2) electrical stress between 14 and 25 kV, (3) normal operating temperatures anywhere between 350 and 9000C with possible temperature changes between these extremes in less than 10 seconds, (4) corrosive environment including bromine, sulphur, phosphorus, and tetraethyl lead, (5) abrasion resistance against high-velocity particles, and (6) reliable operation for at least 30 million cycles.
Can you advise us of potential materials and necessary processing to meet these requirements?
[Based on: L. A. Killick, "The Sparking Plug, Some Design Considerations," Materials & Design, Vol 3, August 1982, pp 511-514]
5. We have been making disk harrow blades using an AISI 1080 steel quenched and tempered to a hardness of 42 Rockwell C. One field test, for example, runs harrows for 250 hours in hard clay ground containing small to medium rocks. Of 28 blades tested, 9 failed. While this is considered acceptable, we would like to decrease the failure rate, if possible. If you can r commend a different alloy and/or process, how would the microstructure compare with that in the above blades? What sort of performance would you "guesstimate" from your recommendations?
[Based on 21/4 column inches on page 220 of Volume 1 of the 8th Edition of the ASM International Metals Handbook. One might also see how this problem has been expanded by Smith in Reference] 20.]