Improved Yield Strength of steel with higher carbon content certain??? 3

[wilmar13]

I tried a search and found nothing... this seems so basic but I can't get a definitive answer anywhere so hopefully someone here can shed some light on the subject.

We primarily make formed and fabricated welded assemblies out of gauge thickness sheet and plate up to 3/8". We use 2 types of commercially available HRP&O sheet and plate, all CS type B in either 1008 or 1018. Most things are 1008, but for things that require more yield strength we spec the 1018.

As I delved into the referenced AISI standard which led to the SAE standard J403 that led me to more standards all of which only defined the chemical composition rather than any mechanical properties.

ASTM A1011 does list typical mechanical property values* of 30-50ksi yield strength and >25% elongation, but also adds a disclaimer: "values outside of the range are to be expected". *-This is for all steels, not specific to carbon content.

So not only do I not know a real min yield strength, but I have no way of validating the belief that 1018 will always have a higher yield than 1008 plate and sheet. Our steel distributor says 1018 HRP&O plate/sheet is custom to us, so that is what got me started down this rabbit hole. This whole 1008/1018 thing for sheet steel was done when there were actual engineers that worked here many years ago, so I am inclined to believe it is correct, but frustrated I can't verify it and that our steel supplier acts like it is some custom thing when the goal was to get the lowest cost steel that will work for the application.

Therein lies my question; 1) does CS HRP&O 1018 ALWAYS have a significantly higher yield strength than 1008 2) If so what is it and how can I quantify it?

I know the obvious is to choose a HSLAS-F steel or something that has a minimum yield well defined for the more demanding applications, but my company is run by accountants and salesmen that are completely clueless and don't know the difference between price and cost, so we save $.05 on materials and spend $1 to make it work.

 

[swall]

It is reasonable to expect HR 1018 to have a higher yield strength than HR 1008, but you can't count on it. If you need a strength guarantee, you have to bite the bullet and specify an HSLA grade.

 

[mpoulter]

Question 1) Generally, yes but, like Swall stated, not always.

Question 2) You can get an approximate yield strength measurement by performing a hardness test on the material. There is a pretty good correlation (plus or minus 15%, usually) between the YS and the hardness of a material. In my line of work, we typically consider the YS to be about 1/3 of the measured Vickers hardness. At the very least, you could compare values for your "custom" material and determine which is stronger.

 

[wilmar13]

I like the hardness test... easy and accessible, and good enough for the gist... the problem is this will never happen, except for intervention. You put the material spec on the print so that it covers all bases and forget it. Perhaps I can use the fact that 1018 is not necessarily stronger in all batches than 1008 as justification to upgrade, but I predict the "we never had problems before" spiel.

We use 1018 for things that need a 35ksi min YS material and 1008 for everything else assuming a 20ksi min YS. I have found 2 samples of the 1008 steel we buy that was tested with a YS of 47ksi! (46ksi for one and 48ksi for the other). Small sample size, but it sure would be nice to know mean and deviation of YS by mill run because perhaps we could just use the 1008 for everything.

Is there anyone that can give me an idea of expected range that the steel mills dance around?

If I had information like, just for example: 1008 has mean min YS of 38ksi and std dev of 5ksi, and 1018 has mean of 41ksi with Std Dev of 3ksi; I could make some informed decisions... right now I suspect we pay extra for 1018 that we don't need... or we pay extra for that 1018 that is still actually underspeced for what we REALLY need; HSLAS-F Grade 50.

 

[TVP]

1018 is not commonly produced in flat rolled products (sheet, strip, coils, etc.), so you are certainly paying a premium for it. Because it is so uncommon, no one will be able to give you mean +/- sd for yield strength, only guesstimates. My WAG for yield strength is 45 ksi +/- 10 ksi. The distribution will certainly be higher than for CS Type B, but there will probably be some overlap. If you need guaranteed 35 ksi minimum yield, then you need to specify either SS Grade 36 or HSLAS Grade 45. FYI, the composition of 1018 according to SAE J403 fits the SS chemical composition limits in ASTM A1011.

 

[wilmar13]

I actually talked to the metallurgist at the Steel Mill last night where most of the material we buy is produced… he told me 1018 is common as well in sheet/plate and is the best low cost option for our higher strength requirements. Perhaps the 1018 sheet/plate is only commonly produced at that mill?

He also confirmed that categorically 1018 will have a higher yield strength than 1008 apples to apples, and also that the 1008 produced in that mill is always near the top of the min YS range in the ASTM spec (which is corroborated with the sample results I have at ~47ksi). Just goes to show that the tribal knowledge that has been lost, was based on something rational after all.

The ASTM spec is too vague and wide, with only material’s mechanical properties defined with high specificity of the steel grade, which is the same steel we already buy, just tested and certified. If we want to be sure that steel made anywhere by anyone meets our criteria we need to be more specific, which means certifications, which means more cost. Just another example illustrating the paradox of global sourcing reducing cost over sourcing things that you know locally; “if we add 10% to the cost of our steel with a more stringent ASTM spec, we can reduce our spend 2% by leveraging our global supply chain.” YAY! I kid but there is more than a grain of truth to that.