Use of 4140 grade steel for structural bolts 2

[Temporaryworks]

I am an engineer practicing in Australia. I have a query about the use of 4140 general purpose high-tensile grade steel as a replacement for a typical/standard grade 8.8 structural bolts.

The context: I have designed an M48 grade 8.8 bolt (880MPa tensile strength) to be cast in to a concrete wall to prevent uplift of a tower crane structural steel grillage (big loads, pre-tensioned bolt). The client is having trouble sourcing the M48 bolt and has asked if they can use a 4140 grade rod and machine the ends to produce threads. I declined their request because I was worried that the grade 8.8 bolt has some properties that the 4140 grade steel doesn't have that make it suitable to be bolt i.e. non-brittle or corrosion properties.

I am not 100% certain that this decision was correct but felt uncomfortable with swapping the grade - could I have people's thoughts on this? Are there special properties which make code-compliant bolts safer than when compared with other steel grades?

 

[r13]

I don't know the steel grade you have mentioned, but generally speaking, the code specified steel has properties verified through standard tests. Other steel might be suitable for some applications, but without been proven through required tests, you are correct in not accepting the swap.

 

[desertfox]

I think you were right to reject the 4140 steel, it as a higher content of carbon than a grade 8.8 bolt steel which means under tensile loading it will behave differently in terms of its elongation particularly in cold weather.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[dik]

Your Grade 8.8 at 880 MPa has a UTS of 127.6 Ksi and the 4140 has a UTS of 95 Ksi... Maybe make it bigger. For anchor rods I often use a material spec and design the AR accordingly. The 4140 is a good material

AISI 4140 alloy steel is a chromium-, molybdenum-, and manganese-containing low alloy steel. It has high fatigue strength, abrasion and impact resistance, toughness, and torsional strength.

Dik

 

[Lomarandil]

Yes, unless it has the properties you need, you're right to decline the swap.

In a pre-tensioned application, I'd be most concerned with ductility/elongation (beyond the Fy/Fu of course).

----
just call me Lo.

 

[Craig_H]

If my memory is right, the AISI numbering system specifies chemical content rather than mechanical properties. I recall being able to source quite a wide variety of mechanical properties depending on the heat treatment of 4140. When using 4140 & 4340 in the past, I always included direct requirements for mechanical properties, including maximum Fy/Fu ratios as mentioned by Lo.

Dik, you seem to have a standard strength of 4140, am I off base with it varying?

It also seems to me that increasing the diameter of the anchor rods with a slightly weaker material would likely result in a stiffer base connection, something that may actually be advantageous, all else being equal.

 

[Rogue909]

4140 designates the chemical composition. Not the mechanical properties. ASTM F568 class 8.8 gives an enormous range on chemical composition (just carbon, sulfur, and phosphorus requirements) but gives a specific mechanical requirement.

4140 *can* meet the chemical composition and can be heat treated to meet the mechanical properties. It sounds like the lot the vendor is offering doesn't meet the mechanicals. Either upsize the part or request a different lot.

For that size part it would be common to upsize to a 'richer' alloy material to get a more effective heat treatment. 4340 material is often common for that size diameter.

If ASTM standards/grades are acceptable; look at grade 7 (or L7) or L43. ASTM A320 discusses the tensile strengths for those.

 

[Tmoose]

fastenal has M48-5 grade 8.8 hex bolts zinc plated in 240 mm and 260 mm long.

And plain finished 150 - 500 mm long.

McMaster Carr has M48-5 gr 8.8 threaded rod ( from China ) 300 mm and 1 m long.

I'd kind of expect Australia has some decent fastener supply houses

 

[dik]

Nope... 880 MPa can be achieved... the substitution is viable... a matter of doing a little reseach... and maybe upsizing... catch the link:

http://www.atlassteels.com.au/documents/Atlas4140.pdf

Dik

 

[rb1957]

the material question can easily be answered. The strength of 4140 depends on the heat treatment. 125ksi is 1/4 hard … very common (95ksi is normalised/annealed).

my problem would be the machine cutting of the threads. I do understand that is how most threads are formed (you can roll form if fatigue is critical). I understand that this is how we tap a hole. I would rather fasteners came from fastener people. They're fasteners, they are the weakest link in the structure, or at least the most concentrated loadpath.

another day in paradise, or is paradise one day closer ?

 

[dik]

rb1957:
Thanks for the added... I knew the 95 Ksi was minimum... but hadn't considered that they are normally supplied stronger. I don't like to skimp on fasteners and the fastener people are the way to go. Again... thanks.

Dik

 

[Agent666]

These documents from NZ from Hera and SESOC offer some advice regarding acceptability of 4140 MATERIAL for HD bolts in larger diameters.

The general consensus seems to be that it is the only material that is a substitute for grade 8.8 on the basis that you account for the obvious differences in ultimate tensile strength. Size diameter to suit, though you may be limited by the hole diameter in your crane base.

Hera DCB 51

Hera DCB 52

Sesoc holding down bolt design guide

Hopefully they are of some help since we share the same bolt related standards.

 

[dik]

Good guide... SESOC?


Dik

 

[Agent666]

Structural Engineering SOCiety, a local (NZ based) professional body/group.

 

[dik]

Thanks...

Dik

 

[Temporaryworks]

Thank you all for your replies, I found them very interesting.

dik, the Atlas excerpt that you attached does mention that it can be used for threaded bolts.

Rouge909, I should check the Australian Standard to see what the permitted chemical composition range is for a typical structural 8.8 grade bolt and see if that matches any of the 4140 sub-grades. My concern was that I had no idea from looking at the material/chemical composition of the 4140 steel if it was suitable for use as a tensioned fastener even though I knew that it was an important consideration.

 

[BJI]

I would expect 4140 material would be suitable (often used for bolting), the chemical composition could be within the limits for class 8.8 or alternatively A193-B7. There will be slight differences in tensile strength but is it that critical for the application? You didn't mention what preload you were taking the bolts to, I would have thought the pre-load would be limited by the strength of the concrete in this case anyway?

 

[Temporaryworks]

Tensile strength is not critical, pre-load is taken to the working load which from memory is about 50% of design capacity