Selecting the Right Steel for the Application

Well, start with a corrosion resistance chart first, Mate. It's no good putting steel in contact with something that will make it rot like billy-o from day 1. Yer better off rubber lining or, better still, making it in plastic. Dodgy Dave in Canvey has got some plastic tanks going cheap if yer wan' em. <Splutter>

Typical mechanical properties for any material selection include, Tensile strength fatige life, compressive strength, wear resistance, rigidity etc. For alloying elements you need to talk to a metalurgist, or find yourself a material database. The material selection process is based primarily on the requirements of the design. Typically with the foundry i work at, hardness is desired for wear surfaces and components, while strength is required for high stress structures, fatigue life for long service designs.

I think you will find though that the main influence on the material selection for most components is availability and familiarity; which is a shame, as it does not give "new" materials a chance for competitiveness and testing.

i want to make a flying platform with its own crane for servicng puting in and taking out oil and gas platforms on or off shorr. Wher shuld i start.

Regards,

Guest quoted: "i want to make a flying platform with its own crane for servicng puting in and taking out oil and gas platforms on or off shorr. Wher shuld i start."

Guest: I would recommend preparing yourself for enrollment in a university educational program leading to a degree in mechanical engineering with some emphasis on structural design. Given your interest in the oil and gas industry and offshore operations some exposure to the subject of marine engineering is also appropriate.

Your mention of "flying platform" suggests a watercraft with a planing or hydrofoil type hull. The design and propulsion of such vessels is a fairly specialized area of marine architecture. Some personal research and study by you should be of immediate interest.

Ed Weldon

A "flying platform" in this case would actually be a cherry picker basket I suspect. I can tell you that no "company man" on any offshore rig operated by any of the big O&G companies in the world would let you within a mile of their rig with a device designed and built by someone who knew so little about engineering or structural design as the gentleman signed in as "Guest". They would all demand to see the welding certs, the design calculations, the material certs and the design drawings before you got anywhere close to their equipment. Because not only is the lives of the operators in danger, but everyone on the rig if the thing collapses and damages the wellhead. There are no "non-critical" pieces of equipment on the rig floor.

Rorschach -- Of course you are right about all this. We're just dealing here with a "kid" who is a dreamer and has in front of him a real world to learn about.

I really didn't expect that lengthy post of mine about application of steels to have much of an impact on his type. What I was really doing was exercising an excuse to try and put the subject of how a design engineer picks what steel to specify into a simple explanation. I wanted to avoid immediately devolving into "ites", phase diagrams and ASTM specifications that can only be read if you are willing to buy or borrow the copies.

My writeup in #3 can still use a lot of editing. So many thanks to you folks who thought enough of it to give me a GA. I'm hoping I get more comments like yours to help me polish that particular piece of my writing.

Maybe someday when I can no longer build stuff (my current project is digging post holes for a retaining wall) I'll put some of these essays I write in CR-4 together into a book of some kind.

Ed Weldon

I would add that low temp toughness may be of concern as well. Many low carbon steels suffer a severe drop off in impact properties (toughness) when the temperature gets at or below freezing. If the product will be used in arctic environments then you will need to consider some of the High Strength low Alloy (HSLA) structural steels that have improved impact properties at low temp. ASTM A-572 Grade 50 for instance is usable down to about -20C. if you need to go lower, you'll have to look at Chrome Moly alloys like 4120 & 4130. ASTM-572 Gr 50 has the added benefit of being weldable using the same weld procedures as conventional mild steel like ASTM A36, but offer 50ksi yeild strength vs 36ksi for A36. The same is not true of the 4120 and 4130.

You should definitely engage the services of either a qualified Mechanical Engineer, or a metallurgist for this.

I am still not clear, which is correct to use (cold or hot rolled steel sheets) for making cylindrical tanks, silos etc. If either of them is ok; would the welding procedure differ for each of them?

So many different different variables go into answering that question it is impossible to say without more information. What will the tank be holding? (water? sand? grain? caustic soda? hydrochloric acid?) what sort of environment will it be in (offshore? middle of the desert? above the arctic circle? in a swamp?). how will it be constructed? will it be under pressure? will ASME pressure vessel regulations apply? and yes, the welding procedure varies quite a bit depending on all of these. that is what welding engineers do for a living.

Dude,

Hire a metallurgist. That's what they do for a living. Or take a class of such kind. What do you think you're going to learn from a forum thread? This is one of the reasons for which American companies were falling behind from a technological perspective-they count too much on people with experience (not to say old) that learned what they know 8 million years ago, and have no technical and theoretical understanding of metallurgy. Some of those companies are finally realizing that. Some of them (oh so many) just go bankrupt with their ridiculous tipical pride and stupidity.

Hi There,

Well I have a question about steel choice and hope you can help me. I am making a guide track for a machine to 'crawl' around the outside of pipework. the pipes could be as large as 108 inches in diameter, and the track will have a tooth profile laser cut into one side, with various holes cut into the steel. I am looking for a steel that will cope with low temperatures can be heat treated to a spring temper but resiliant enough that it can take the occasional tap with a hammer. (it should be positioned by the use of a soft face hammer, but i guess someone will hit it with something a bit harder!) Although the band or track will be potentially exposed to low temperatures (-30 possibly the lowest) the pipe will always be pre-heated to between 100 -140 degrees centigrade before the machine will be put on the band.

I appreciate the comment by one of the other posters about speaking to a metallurgist, and I have spoke to 2...... they both contradicted each other, and then the internet contradicted both of them..... HELP!!!!

Currently am looking at CS70 then heat treating it.... but just wonder what you guys thought!

I wonder if anyone could offer a suggestion on the steel grade I should look at. It will not be welded!

Leonardo -- Being from the USA I had to look up CS70 to find that it is an annealed cold rolled steel similar to our AISI 1070. I'm not a metallurgist, just a mechanical design engineer. But it seems to me that your choice of a plain carbon steel at high hardness for a service temperature of minus 30C is risky. And your offhand "heat treated to spring temper" tells me you haven't looked closely at the charpy V notch impact test data. What you would see in the impact test curves for energy vs. temperature is a pretty sharp "knee" in your temperature range and an order of magnitude of variation depending on the hardness of the steel.

Beyond that I would reconsider the need for "Spring temper" type hardness (50-60 Rockwell C as I would view it) for your application. If this hardness is to provide wear resistance for your track I'd hasten to point out that the process of laser cutting "teeth" in the track does not produce a high quality tooth form and surface finish. So such a condition may produce excessive wear of the mating drive part in the "crawler" mechanism.

As an engineer I would run, not walk, away from the plain carbon steel selection and seek a steel alloy with chromium and molybdenum or nickel content like our AISI alloy 4140. I suspect the unknowns caused by scatter of impact test data for CS70 type material would force you into an expensive quality control program for manufacturing your tracks that likely would dwarf the cost of going to a better suited steel alloy or force your client into maintaining financial reserves to cover future failures under warranty. This suggests to me the reasons why you have already encountered conflicting recommendations for your material selection.

One other factor needs to be considered. That is the basis of the minus 30C worst case exposure condition. If this is based on some set of past weather data for the equipment location you should note the current opinions of experts in the subject of climatology and be somewhat wary of assumptions of future conditions. It is possible that in the future the UK could experience new record low temperatures in spite of the general trend toward global warming.

Ed Weldon

I wish to thank you both for your comments and advice. There is a lot to think about there and I have started to speak to my manufacturing guys to try and come up with a long term solution. I wish I could employ you guys!!! You are right about the mating part on the crawler, but it is sacrificial and is a very cheap, simply changed part. Although in tests I have seen some wear which happens quite quickly to a point, then seems to stop. The current track and gear combination has covered a couple of hundred runs and still not showing any further wear. The final target would be to cover 100 runs between drive gear changes. and I know that as the hardness of the track changes, then so will the wear characteristics. as long as I get 100 passes then I am happy!

I am going to speak unto the metallurgists again today and see what help they can be in light of your comments. I will let you all know what is said. once again thank you all

instead of laser cutting the teeth on your track, consider Wire EDM. the surface finish won't be as good as a hobbed and ground gear tooth but it will be better than laser cut by a good bit. a 32uIn finish should be obtainable, especially after QPQing the part which improves surface finish as well. You can also have the part electropolished which will turn a 32 finish into a 16 finish without physical machining.

Hi There again all, and once again, thank you for your input. OK, I have thrown this at the manufacturing engineers with specifications then it is not my fault! Haha

I have asked the question of wether edm is available for the teeth, and am waiting for the factory to come back to me. I am a wee bit concerned that my choice of base material will be limited if I fail to specify an exact specification as the factory is in China and if I just ask for steel, then the only thing I can be sure of will be the fact that it will be some form of steel! The engineering quality of the factory is absolutely excellent, but material specs concern me! After all, I will not know if the material is inferior until my customers complain!

I will let you all know what happens! thank you!