It is known, laser irradiation increases retained austenite content in many steels ([3] and fig.1). Next effects were investigated as possible reasons of this phenomena: a high cooling rate; a higher peak temperature compared to furnace hardening under standard conditions; a short austenizing time; grain refinement; greater strain hardening of initial austenite before the martensite transformation starts; influence of plastic deformation and stresses; nitrogen pick-up from the atmosphere during irradiation; up-hill diffusion of carbon.

We can draw the following conclusions concerning the explanation of the increased rA content after laser hardening (LH) of furnance hardened carbon and low-alloyed steels [1, 2]:

1. At relatively low peak temperatures (low beam energy), the increase rA content is due to the effect of an increased dislocation density which originated in the initial austenite during reverse polymorphic transformation and which is not recovered when the martensite transformation starts because of a short interaction time.
2. At high peak temperatures near the solid-liquid interface phase region (high beam energy), the increase rA content is due to the higher amount of carbide dissolution compared to furnance hardening under standard conditions.
3. An increase of the peak temperature and/or an increase of the pulse duration weakens the first mechanism and favours the second one. This weakens the influence of the laser processing parameters on amount of rA.
4. The effects of dislocation density and carbide dissolution are superimposed by a third mechanism: this is the up-hill diffusion of carbon resulting in an carbon enrichment in the near surface layers which causes an decrease of M_S.

Some more findings on the rA in irradiated steels:

1. Effect of carbon on the rA content (fig.1, [3]).
2. Breakdown of the rA against tempering temperatures (fig.2, [4]). As can be seen, the relative stability of the rA (δA parameter) is not affected by type of the hardening (LH or furnace one).
3. The rA content in irradiated 1.0%C-1.5%Cr-Fe steel decreased with 40…42 to 35…37% after 3 years holding at room temperature [4].
4. The structure broadening of rA lines depends near-linearly on the amount of the γ-phase in the steel (fig.3, [5]). The type of heat treatment (furnace hardening or LH) and the concentration of carbon have not direct effect on substructure parameter, but do their action through effect on rA content only. There are departures from such dependence (when heating to the intercritical temperature range and when melting).

Fig.1. Effect of carbon on the retained austenite content in steel after laser hardening (LHc - complete, LHi - incomplete laser hardening) [3]; FH - after conventional furnace hardening, literature generalised data.

Fig.2. Breakdown of retained austenite at various tempering temperatures (for 2 h). A - content retained austenite in steel; δA - relative amount of brokendown retained austenite: δA = (Ai − A) ⁄ Ai×100%, where Ai - initial content of retained austenite, %. 1 and 3 - 1.2%C, 4 - 1.0%C, 5 - 0.8%C steels, 2 and 6 - 1.0%C-1.5%Cr steel; 3 and 6 - after furnace quenching, the rest - after LHT, 7 - relative amount of brokendown retained austenite (δA)

Fig.3. Dependence of the structural broadening of lines (200)γ, the size of mosaic blocks and microdistortions of the crystal lattice of retained austenite on its content in steel (the solid line - triple term approximation, dotted one - linear approximation). Symbols in the graph: LSM - laser surface melting; LHc - complete and LHi - incomplete laser hardening; FH - furnace hardening; FH2 - double furnace hardening; Temp. - tempering temperature [5].

1. S. A. Fedosov. Laser beam hardening of carbon and low alloyed steels: discussion of increased quantity of retained austenite // Journal of Materials Science, Vol.34, issue 17, September 1999, pp.4259-4264.
2. S. A. Fedosov. Analiz prichin uvelichenija kolichestva gamma-fazy v poverkhnostnykh sloyach uglerodistykh staley pri lasernom obluchenii (The analysis of causes for gamma-phase quantity increasing under laser irradiation of quenched high-carbon and low-alloyed steels // Fizika i Khimiya Obrabotki Materialov (Physics and Chemistry of Materials Treatment) 1995, n.3, pp.40-48. (in Russian)
3. S. A. Fedosov. Effect of laser treatment on the retained austenite content of carbon and chromium steels // Physics and Chemistry of Materials Treatment (English translation of Fizika i Khimiya Obrabotki Materialov) 1990, Vol.24, n.5, pp.441-444.
4. S. A. Fedosov. Stabilnost ostatochogo austenita posle lasernoy obrabotki staley (Stability of residual austenite after laser treatment of steels) // Fizika i Khimiya Obrabotki Materialov (Physics and Chemistry of Materials Treatment) 1991, n.3, pp.141-142. (in Russian)
5. S. A. Fedosov. X-ray diffraction analysis of the substructure of retained austenite after laser treatment structural steels // Physics and Chemistry of Materials Treatment (English translation of Fizika i Khimiya Obrabotki Materialov), 1992, Vol.26, n.1, pp.98-102.
6. Program "Thermo" was used.