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I have a curiousity question regarding the various steels and strengths of each used in crankshaft and connecting rod forgings. It seems like these numbers get used as buzzwords in the engine builing lingo. In particular, there are differences in the steel types used in original GM cranshafts and current day builders. I don't know if there is any credence in the idea that lower numbers are weaker and higher numbers are stronger, but, I know some GM steel crankshafts were forged from 1053 steel (some of the original steel cranks from the 60's and the current LS6 BB and 502 cranks. I happen to have a 427 7115 forging I am using in my 66 L72 project, which is constructed of 5140 steel I believe. It seems most of the current aftermarket crankshafts are constructed of 4140 or 4340 steel. So, how do all of these various materials really stack up? Is the grade of steel used in my 7115 better/stronger than the current 4340 steel crankshafts? I realize there are many variables determining the overall strength of a forging than just the material, but I am curious what the attributes and advantages are of each of these materials. I am hoping Duke or John chime in!...thx, Craig
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Re: Steel Crankshaft Metallurgy Questions
Boy I miss the spell checker! I guess I should start previewing my posts... -
Re: Steel Crankshaft Metallurgy Questions
do not worry about the strength of you stock GM BB crank shafts as i have built 482 and 510 cu in blower boat motors that make close to 800 HP on pump gas and have been running for 10 years. using a engine in a boat is like putting it on a big dyno,the water. always use new parts because you never know how many cycles are on the parts if they are used unless you have had them since new.Comment
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Re: Steel Crankshaft Metallurgy Questions
Craig-----
The "numbers" used in describing the different steels represent the identifying number of the steel alloy which is used. They are NOT some sort of index of strength with greater numbers indicating relatively greater strength. I've forgotten which organization assigns the numbers and specifications (e.g. ASTM, SAE AIIS, etc.), but I know the information on the steel specs is contained in the Chemical Engineer's Handbook, among other references.
The various steel alloys have a wide range of different properties, including several different parameters relative to "strength". In selecting steel for different applications, the steel alloy which has the best OVERALL COMBINATION of attributes for the specific application is usually chosen.In Appreciation of John HinckleyComment
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Re: Steel Crankshaft Metallurgy Questions
Craig,
I don't have anything to offer beyond what's been written. Here's a site that explains, in every way you would ever care to read, about what the numbers mean: http://www.principalmetals.com
Now, it doesn't say what crank (or rods) you should use in your engine but there were plenty of engines that made a lot of reliable power on 1010 steel and even nodular iron cranks.Comment
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Re: Steel Crankshaft Metallurgy Questions
Thx everyone for the great input!!...CraigComment
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Re: Steel Crankshaft Metallurgy Questions
The "hardness" of steel, which is a surrogate for it's yield tensile strength (YTS) and ultimate tensile strength (UTS) is a function of the carbon content, which is typically in the range of 0.1% to 1.0%. The hardness of carbon steels can be increased with heat treatment, and the greater the carbon content the harder the finished product after heat treatment. Heat treatment generally involves heating the finished part to cherry red, then "quenching" it by rapidly immersing it in water or oil.
As carbon content rises, the hardness that can be obtained by heat treating increases, but ductility is reduced - which is represented by the difference in YTS and UTS. Low carbon steels, which are very ductile are used for forming sheet metal parts and machining parts that don't require high UTS. Low carbon steels form and cut very easily.
Medium carbon steels - say 0.3 to 0.6 percent carbon - with final heat treatment - are used where high strength and reasonable ductility is required. High carbon steels - say 0.7 percent and up are used where very high hardness is required, such as cutting tools, but they lack ductility. If you bend a razor blade, it will usually just snap instead of deforming like a piece of low carbon steel.
Plain carbon steels are usually specified by a four digit number, and the last two digits represent the carbon content in hundreths of a percent, so SAE 1010 and 1020, have 0.10% and 0.20 percent carbon, respectively.
Now the question: Why alloy steel, which brings to the party small quantities (usually less than one percent, rarely more than two percent) of other metals such as chromium, cobalt, nickel, molybdenum, manganese, tungsten, and vanadium and other trace alloying elements. The reason is that the strength derived from heat treatment is a function of section size. Heat treadtment only creates uniform properties across small sections - no more than an inch across. By adding alloying elements the increased hardness from heat treatment penetrates deeper and makes a stronger part. This is why some crnakshafts are forged from alloy steel, but I question the need for alloy steel for connnecting rods, because they have relatively small section widths. In many cases, I believe that rods forged from 4340 (chrome-nickel-molybdenum, 0.40% carbon) are probably, at best, only marginally better than 1040. Both these materials are "medium carbon" steels that can be heat treated to relatively high hardness while maintaining good ductility, which is good for fatigure life.
There are many alloy steels that have been developed over the years and each has a variety of properites including strength, ductility, formability, ease of machining, weldability, fatigue resistance, hardenability, and of course, cost. A designer and materials engineer usually work together to specify a material that results in the best finished part to meet the requirement at the lowest possible manufacturing cost.
DukeComment
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Re: Steel Crankshaft Metallurgy Questions
Duke - EXCELLENT information! I have cut and pasted it into a file for future reference. So, does this mean that the 5140 steel used in my 7115 forging crank is 0.4% carbon, but what does the 51 mean versus say the 43 in the first two digits? It appears to realate to other alloys in the steel besides carbon? It must be since GM used the 5140 in their best cranks that these have more ability to withstand severe loading than the other 1053 units? Thx again!....CraigComment
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Re: Steel Crankshaft Metallurgy Questions
51xx is a chromium alloy steel; 43xx is a nickel, chromium, molybdenum alloy steel, and 5140 would have 0.40% carbon which would heat treat to a fairly high hardness indicating medium-high UTS while maintaining good ductility and excellent fatigue resistance. I can't give you a detailed rundown on the specific properties of 51xx vesus 43xx, but the 51xx is probably a bit cheaper while having many of the favorable properties of 43xx in a high cyclic loading environment.
Because of the two plus inch plus section thicknesses of crankshafts, the hardness due to heat treating would penetrate further on 51xx than a plain carbon steel like 1053, so overall, a 5140 crank would probably be tougher than a 1053 eventhough the latter's higher carbon content would allow a greater hardness near the surface.
Bottom line is that ANY Chevrolet forged steel, nitride surface hardened crankshaft is essentially INDESTRUCTABLE in a street engine as long as the bearings don't get oil starved.
DukeComment
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P.S.
5160 is a common spring steel, so I would opine the a high chromium alloy steel like 5140 would impart greater fatigue resistance than a plain carbon steel with similar carbon content.
DukeComment
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Re: Steel Crankshaft Metallurgy Questions *TL*
The body that sets the standards for steel composition is AISI - American Iron and Steel Institute - in conjunction with (Ta Da) SAE - Society of Automotive Engineers.
Here is a link to a chart that contains more information.
TerryComment
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Re: P.S.
Duke - THX again! I truly appreciate the input. I saved it again. My crank will be ground during the rebuild, so I suppose I am loosing the surface nitride treatment. But, since I am building the engine to stock L72 1966 specifications including the pistons and cam, I suspect I don't much need to worry. Thx again, CraigComment
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THX Terry!...... *NM*
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Re: P.S.
I would recommend not grinding the crank unless it's absolutely necessary.
The forged steel tuftrided crankshaft on my '63 L-76 spun freely in the main bearings on engine disassembly, and later measurements proved the main bearing saddles to be in perfect alignment, so I knew the crank was straight. All the journals miked to about the middle of the OEM dimension range and it passed a magnaflux test. I had the journals polished and it was ready for another life in standard sized bearings.
Grinding the crank will remove the surface tuftriding, but it probably won't impact longevity in a street engine. Just the same, don't grind it unless absolutely necessary. Machine shops love to remove metal, but often it's not necessary.
DukeComment
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Re: P.S.
Thx for the advice Duke, but in this case I have no option. It was stored and has some surface rust, and won't clean up with just a polish, and it is already at 10-20 anyway. However, my machinist is a finiky type, and builds lots of race engines and is the blueprint type of guy. He has a grinder he likes, and wants to insure the journals get a nice radius on the edges too. I thought about getting it re-tuftrided, but, he said that would probably be quite expensive if I sent it to a crankshaft supplier. I am hoping since this is an nice, crack free, 7115 5140 L88/Zl1 piece, it will serve me well in my stock build up 66 L72 engine. I am also using the GM dimple rods with 7/16" bolts, otherwise, the rest of the engine will have the Speed Pro L2268F pistons, 0.030 over, and I intend to get a Crane (through GM 3833143 copy) or, I understand Federal Mogul (now Speed Pro?) has a blueprint grind of this cam. Anyone know the number? Thx!....CraigComment
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