Prakt. Met. Sonderband 38 (2006) 393
As stated above, an increase in thickness reduction caused the time of etching to reduce.
This is basically due to increase in density of defects populations such as dislocations.
High dislocation density provides an extra effective energy particularly for the surface
atoms exposed directly to etchants, by which the rate and driving force of corrosion at the
surface increase, so etching time expected to decrease. As it is shown in Figure 1,
reduction of etching time with increasing the amount of deformation when using Fry,
Aquaregia or Hydrochloride acid is linear, but this is not the case for Glyceregia. The rate
of reduction of etching time with increasing deformation when Kalling used decreased in
comparison with the above mentioned solutions. Effect of Nitric acid on the sample 8
presented in table 5 was in such a way that the time of etching, by increasing deformation
pm decreased approximately linear, but for sample 13 the rate of etching time decreasing was
— in an exponential way with increasing the amount of cold work. So one may conclude that
the amount of cold work not only affected the etching time but also the strength of the
solution used and even the slight difference in the compositions of the samples (i.e.
samples 8 and 13) can effect the etching time.
The quality of the pictures obtained by optical metallography depends not only on etching
solution and the technique used but also to composition of the steel. Slight variations in
composition affect the microstructure clearance under an optical microscope. The
differences in microstructure revealed by various etchants and techniques are shown in
figure 2. Looking the revealed features one may recommend the following etchants and
techniques for different metallographic purposes.
Table 7: Recommended etchants and etching techniques for revelation of Sandvik
stainless steel microstructure
= Etchant Technique Type of Microstructure
Aquaregia Soaking General Microstructure
Fry Soaking Martensite
Glyceregia ! Soaking Grain and Twin Boundaries
Kalling Soaking Martensite
Nitric Acid Electrolytic Grain Boundaries
Hydrochloric Acid Electrolytic Grain and Twin Boundaries
Orthophsphoric Acid Electrolytic Twin Boundaries
V2A Hot Soaking General Microstructure
4. Conclusions
1- Increase in amount of cold work for the same samples causes the etching time to
jum decrease.
a 2- Thermal etching in air on Sandvik 1RK91 has no considerable results.
3- For revelation of general microstructure of Sandvik 1RK91, etchants such as Aquaregia
3: and V2A are effective. V2A recommends for martensitic microstructures.
a 4- For revelation of pure grain boundaries of Sandvik 1RK91, Nitric acid is the best etchant
while Aquaregia is effective sometimes.
ce 5- For revelation of pure twin boundaries of Sandvik 1RK91, Orthophsphoric acid is the
best while Hydrochloric is suitable for some compositions.
ad in furnace 6- Martensite in martensitic microstructure revels by Fry and Kalling reagents, however
Kalling results in better contrast.