I am in the process of obtaining a recent reference that discussed small additions of Cr to a high carbon steel. From what I remember, Cr additions in the range of 0.2-0.3 % were helpful to cold workability, because the pearlite spacing was reduced. I'll get back to you when I have the reference.
Ok, here is the scoop. Small Cr additions, in the range of 0.1-0.3, increase the formability/reduction of area of high carbon steels by promoting the refinement of pearlite (reducing the interlamellar spacing, [ignore]ë [/ignore]). Now, this applies to high carbon steels where the microstructure is pearlitic-- cementite distributed as fine alternating lamellae with ferrite. Cr delays the transformation kinetics, thereby reducing interlamellar spacing, but additions greater than ~ 0.3 causes the initially-formed cementite to undergo partitioning and begin spheroidizing.
If the steel is annealed, then this may no longer apply, depending on the time and temperature used. If the structure is spheroidized, then the Cr may have little or no effect on formability, due to the structure being composed of ferrite and spheroidal carbides, not pearlite. You need to ascertain whether or not the steel in question truly has been annealed. Two excellent references on this subject are the following:
SAE Technical Paper 2003-01-1179, Developoment of 1600 N/mm2 Class Ultra-High Strength Bolts by Takashima, et al.
In the SAE paper referenced by TVP, the authors equated formability with reduction in area r. Changing Cr concentration from ~ 0 to ~ 0.2 weight percent changed r from 0.49 to 0.56. The equivalent true strains for these r values are 0.67 and 0.82. I wouldn't call this a "drastic" change, but it certainly is a good one. Also, these data are for tension behavior, but compression behavior may not see the same benefit. Therefore, "cold workability" may or may not be influenced by Cr addition depending upon the tension and compression strains imposed during forming.
Apparently you must buy these papers to read them.
So, without having read them, I have to ask this question: True, Cr can delay the transformation of austenite to pearlite, thus causing the pearlite to be finer. But so can Mn and Mo to approx. the same degree. Unless something new has been discovered, that slight % of Cr could easily be offset by a slight reduction of Mn, or a few other elements. Also, if heat "A" and 0% Cr was cooled somewhat faster than heat "B" which had a little Cr (all other elements the same), it could easily have finer pearlite spacing than heat B.
Yes, one must purchase an SAE paper if one does not have access otherwise (public library, company subscription).
In this SAE paper, the steel had a Mn concentration of 0.6% - 0.9%. Thus, 0.2% Cr addition will be more effective than an additional 0.2% (or more) Mn addition. Your points regarding composition and cooling rate fluctuations are correct, but I think the authors were looking to keep these conditions (relatively) constant and determine what composition gives the best properties. The authors found that Mn (a ferrite solid solution strengthener) strengthened the steel, but also reduced formability. There was no mention of Mo in the article. It doesn't seem like a big deal to choose either Cr or Mo to add at the 0.2 weight percent level.
