impact toughness

[vukk]

Hi,
I want to ask you for your opinion.
I have such steel: 0,23%carbon, 1,3% manganese, 0,2% silicon, 1,0% nickel, 0,55% molybdenum, 0,55%chromium and this steel is usually rolled to bars.
This material has high strength and should have the better impact toughness (determined by impact energy).
However the impact toughness is not satisfactory.
I tried to decrease the finish rolling temperature to get a fine grained structure to get a higher impact toughness but without avail.
Do you have any idea, how to increase the impact toughness?
Thanks

 

[IRstuff]

What do you think it should be and what are you actually getting?

TTFN

FAQ731-376

 

[MikeMet]

I do a lot of work with high strength low alloys steels used in high pressure applications. The more Mn, Si and S in the mix the more difficult it is to get satisfactory CVN even in fine grain strucutures. For our applications with a very similar alloy we restrict Mn to 0.85 max, Si to 0.3 max and S to 0.025 max. Other trash elements, such as copper, can also have detrimental effects on impact strength. Using a normalize, Q&T heat treatment we routinely get greater than 35 ft-lb at 0 deg. F.

 

[CdotS]

Vukk, please provide of actual CVN values as IRStuff says. Also, some info on the process history such as heat treatment temperature and time and microstructure would be helpful. Generally, tempered martensite has higher toughness than as rolled pearlitic/ferrite.

MikeMet, what is your hardness when you measured a toughness value of 35 ft.lb at 0 deg F?

Thanks.

 

[vukk]

Thanks for replies,
Process history: 1) Quenching (880°C temperature/water)
2) Tempering (360°C temperature/ 1 hour/air).
Microstructure: tempered martensite
Impact energy: approximately 20-30J and we need 40 - 50 Joule.
And I can't lower the tempering temperature and have only a narrow possibility to increase the tempering temperature by reason that it will decrease an yield strength and tensile strength of this steel.
Thanks

 

[CoryPad]

What are you P and S concentrations? They need to be as low as possible, e.g. < 0.020 percent by mass.

You need small austenite grain size. Can you use Si and Al in your melting/deoxidation/casting process?

What strength level are you trying to attain?

 

[redpicker]

As CoryPad mentioned, the needed strength level would be helpful. You say you need 40-50 Joules, at what temperature?

Do you have a minimum of 40 Joules you have to meet, or do you need to be between 40 and 50 Joules (that is, 55 Joules would be unacceptable). This would be very unusual.

The tempering temperature of 360C is very low and could be contributing. With 0.55% Moly, you might get some secondary hardening effect and be able to push that tempering temperature up to the 540-600 C range, which should put those impacts in the 80 joule range. If you cannot hold the strength at those tempering temperatures, go up in carbon.

rp

 

[vukk]

I need minimum 42 J at the temperature -40°C.
This is quit difficult because I need a tensile strength minimum 1400 MPa in the same time.
By pushing the tempering temperature up to 540-600 I don't get required strength even after increasing a carbon content.
I found the tempering/strength limit at 450°C.

 

[CoryPad]

How about keeping carbon low (for high toughness) and adding boron for hardenability?

 

[vukk]

to CoryPad
Yes, I will try to lower carbon content but I'm looking for an another way.
With boron we have a bad experiences because this material is welded by using flash welding. And the weld joint isn't good.
Thanks

 

[TVP]

vukk,

This is quite a challenging problem-- very high strength, high toughness at low temperature, and a weldability requirement. You have received some good information from the others so far, but I wanted to add my own comments, some of which have already been mentioned:

1. S and P must be minimized. The recommendation of 0.020 % (by mass) maximum that was previously mentioned is still probably too high for this combination of requirements. I recommend 0.010 % maximum.

2. Don't use silicon deoxidation and don't intentionally add Si to this alloy. Toughness will be improved if you limit Si to 0.10 % max. Al deoxidation is preferred for these types of applications.

3. The prior austenite grain size needs to be very small, meaning below 30 micrometers. Decreasing the final rolling temperature (thermomechanical rolling) is a very good idea and preferably below 850 C.

4. Even with these improvements you may not be able to achieve what you want with this chemical composition. The low toughness occurs due to the precipitation of thin cementite films along the prior austenite grain boundaries. This occurs at low tempering temperatures such as the 360 C that you have previously used. Tempering at higher temperatures such as in the range of 500-600 C spheroidizes the cementite so that it no longer forms grain boundary films (semi-continuous networks). Unfortunately the low C content you have prevents such high temperature tempering. You would be better off increasing the C content to 0.35-0.40 and increasing the tempering temperature. One excellent example of this was presented by S. Kato et al from Daido at the Asia '06 steel conference: Delayed fracture resistance of high temperature tempered NiCrMoV steel for bolts.

 

[vukk]

To TVP - Thanks, do you have this paper in english?

 

[salmon2]

This is very diffcult requirement to meet. I would like to know your results eventually. Other people have pointed out almost all the possible directions, you have to try them all and find the best combination of strength and toughness, you may still not achieve this. I will provide something I know just for discussion purpose:

1) S is much more detrimental to tranverse toughness rather than longitudinal direction. So if you only care about longitudinal toughness, probably .020% max is fine.
2) Can you afford addition of Ni and Mo which will promote strength, low temperature toughness and secondary hardening. This chemistry is a little bit cheap to me for this application.
3) As many people pointed out that your tempering temperature is too low. I know that was how you got 1400 MPa yield strength, but it is also why you have low Charpies. So I am thinking maybe you can add more Mo and take advantage of 2nd hardening effect with high tempering temperature.
4) Tailor your heat treatment cycles to get finer grain is always good to go.

 

[salmon2]

I have overlooked weldability in my previous post, do you want to work on it afterwards? Adding Ni and Mo worsens welding in general.

 

[TVP]

vukk,

Yes, I have the paper in English.

 

[vukk]

TVP,
I found only an original in Japanese.
Could you give me please a link on this paper in English?

 

[TVP]

vukk,

The paper is not freely available online. I suggest you contact the authors or try to find the conference proceedings.