stanislasdz;
This is not an easy post to answer because there are many variables that effect notch impact and toughness behavior of carbon and low alloy steels – chemistry and heat treatment. If you really want to know the specific effect, you need to conduct your own mechanical property testing on the steel of interest and draw your own conclusion.
I have done some work with CVN impact qualification of various grades of plate steels hot rolled and normalized (SA 285, SA 516 Grade 60, 70). From the technical literature I have seen specific to general carbon and low alloy steel and for the same grade of steel having a range of tensile YS/UTS values , lower YS/UTS ratios promotes greater ductility. The greater ductility favors larger plastic zones and blunting of notches or cracks, which would increase notch toughness and CVN impact values, in general. I have not seen any specific correlation to describe this behavior; it is only based on my past work with testing of pressure vessel plate steels.
Metengr is correct that there are really too many variables to come up with any good correlation. However, with high strength Q&T steels, I've found the YS/UTS ratio to correlate well with the amount of martensite formed on quenching; the higher the ratio, the higher the amount of martensite. Since, in general, the more martensite formed on quenching the higher the impact strength, there is a correlation. You do have to limit the varriables, particularly chemistry. But, for two Q&T samples of the same steel grade at the same tensile strength levels, the one with the higher YS/UTS ratio will generally have higher impact strengths and lower transition temperatures. Of course, there are other factors to consider, but if you control these other factors, you can find a decent correlation. For example, with 4140 Q&T to 30-35 HRC, if you have a YS/UTS ratio above 0.85 or so, I'd expect impact strengths around 50-80 Joules (40-60 Ft-Lbs). At the same hardenss, but YS/UTS ratios below 0.80, impact values will be more in the 20-40 Joule (15-30 Ft-Lb) range. Like metengr, this observation is only based on my expereince and I really don't have any specific explanaiton for this behavior.
I find it very interesting that this is in direct oppositon to metengr's observations. Although my experience with mechanical properties of normalized steel is low, I understand and agree with his comments and explanations. I think the differences result from dealing with a ferritic/pearlitic microstructure as opposed to a martensitic/bainitic microstructure.
There is no general correlation between YS/TS and Charpy-V-values.
Some arguments, that high YS/TS values correlate with high Charpy-values:
a) A finer ferritic grain size results in higher yield strength (Hall-Petch-law) and in higher impact properties. With the same chemistry, a normalized A516-60 has higher YS and higher impact values compared to an as rolled.
b) Mn has a positive effect on YS and impact properties (as long as a ferritic structure is obtained). C has the opposite effect. E.g. 2 plates with same TS in A516-70
one with high C and low Mn, one with low C and high Mn. Ductiliy and YS/TS are higher for the second plate.
c) Very high Charpy-values can be obtained in TM-rolled X65 steels with very low C-content, e.g. 450 J at -60°C. YS/TS about 0,90. I have seen Q+T HSLA100 plates with YS/TS of 1,00 (even slighly above if upper yield strength is measured) and Charpy-values above 200J at -40°C. On the other side you may find steel grades with low Mn-contents (0,3%) with YS/TS of 0,50 and very poor ductility, even at room temperature.
d) There are influences that strongly effect Charpy-values without having a substantial influence on YS, like non-metallic inclusions, hydrogen content, laminations.
But the contrary is also existing:
e) Bauschinger may reduce the YS and so YS/TS without any effect on TS and impact values
f) Cold deformation rises YS and YS/TS and lowers ductility.
Metenger : Like redpicker from my database for Q&T steels when YS/UTS is lower then 0.85 we have lower values of CVN and when this ratio is higer than 0.85 we found best values of CVN.
