metallurgy question for blakey

General Pub
1

hey dude. was my texto a bit random? got your number off nath… he told me you knew about alloys & stuff.

anyway, what do you reckon? does heat-treated steel like 853 fatigue if it’s stressed within its elastic limit? ta mate. :smiley:

2

Nah

3

OK, so I guess the question is then: Is the Fatigue limit for heat treated alloys of steel (like 853) low enough that the stresses of cycling exceed it?

Someone has proposed that only mild steel has a high enough fatigue limit, whereas 853 and friends will fatigue much like aluminium. That didn’t seem right to me, but I don’t know the facts.

4

I’m glad you bought mine then. It’ll probably break soon.

5

I’m glad you bought mine then. It’ll probably break soon.

hahah - it’s only has to last until i get a Baum replacement. But seriously - there’s no way that frame is going to break anytime soon.

6

It was a little random, but not too random, I hadn’t gotten around to replying yet though, been a little busy with moving and approaching my own fatigue limit.

I have a bunch of photocopied S-N (*1) curves at work for 4130 (*2) and similar. I thought I’d finished up last Friday, but I guess I could go back in and have a look at them for you so we can have some actual numbers to discuss.

PS, nath, got your fullySiC paper in the car being trucked to Melb. 120 up to 4000.

*1 S-N is stress vs number of cycles.

Al vs Steel (flat line = fatigue limit, Ti also has one.):

*2 4130 ~ 531.

7

NEVER TAKE ON BLAKEY…

8

Well, at least with regards to materials science…I wouldn’t bet on me at a bar fight.

TLDR’s skip to the end…

Anyway, from Boyer’s ‘Atlas of Fatigue Curves’, here’s a few tidbits.

  • Tempered martensite was considered to have optimum fatigue properties (you get martensite when you quench steel, i.e. go from very hot to cold very fast, increases strength a lot, reduces ductility. Then you temper it to get some ductility back.)

  • Better surface finish = better fatigue properties. So grind those weld beads nice and smooth, shot peen the surface, file out stress concentrations.

  • Even though steel has a fatigue limit, corrosion can nullify it.

  • unnotched 4100 steel samples have a fatigue limit varying between 500 and 900 MPa, where the hardness varies from 20 to 60 Rockwell C.

  • For a few medium carbon steels (1340/4042/4340/5140/80B40) with similar strength levels (36 HRC), the fatigue limit is 600-640 MPa at 100% martensite and 475-550 MPa at 10-50% martensite.

  • For a mild steel (low carbon, not much else), the fatigue limit is ~250MPa.

  • Conclusion: 4100 ~ 500-900MPa fatigue limit, mild steel, ~250MPa fatigue limit

Don’t think that a gaspipe bike will have better fatigue properties because it’s not highly alloyed. It’ll probably last longer because it’s about 100x overbuilt vs say 10x for a nice light 853 frame.

9

wowzorz.