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Heat treatment of long term serviced Cr - Mo cast steel

Golański G., Kupczyk J., Kolan C.

Archives of Foundry Engineering

2010

Vol. 10, iss. 1

45-50

The paper presents results of research on the influence of heat treatment on the structure and properties of L20HM cast steel after long term operation at elevated temperature. Investigated cast steel was taken out from an outer frame of a steam turbine serviced for 167 424 hours at the temp. of 535 °C and pressure 12.75 MPa. In post-operating condition the investigated cast steel was characterized by mechanical properties below the required minimum and by high brittleness. Performed research on the influence of austenitizing parameters has revealed that the range of austenitizing temperatures for the examined cast steel: A_c3 +30 ÷ 60 °C ensures obtaining of a fine austenite grain, homogeneous in size. It has been proved that tempering of bainititc - ferritic structure above 680 ÷ 690 °C causes an increase of impact energy along with a decrease of mechanical properties below the required minimum. Moreover, it has been noticed that applying of under-annealing instead of tempering, after full-annealing, guarantees the required impact energy of KV > 27J, with the mechanical properties similar to those after service.

Influence of austenitizing parameters on former austenite grain size in Cr-Mo cast steel after long-term service

Golański G., Kupczyk J.

Inżynieria Materiałowa

2008

Vol. 29, nr 4

266-270

The paper presents results of research on the influence of austenitizing parameters, temperatures in the range of 895^-985°C and times of 3 and 5 hours on the size of former austenite grain in G20CrMo2-5 (L20HM) cast steel. The research was carried out on samples taken from long-term serviced (around 160 000 hours) steam turbine frame. Description of the influence of austenitizing parameters on the size of former austenite grain was made by means of diameter and area of the grains. Performed research has proved that: distributions of diameters and areas of grains are lognormal in character on the significance level of a=0.01. The diameters and areas of grains in the following range of austenitizing temperatures: 895-^- 985°C and for 3 and 5 hours of holding time, do not reveal any crucial differences. Significant grain growth has been noticed at the temperature above 955°C. Moreover, it has been shown that for the investigated cast steel the optimum range of austenitizing temperatures is Ac3 + 30-60°C.

W pracy przedstawiono wyniki badań wpływu parametrów austenityzowania: temperatury w zakresie 895-f-985°C i czasów 3 i 5 godzinna wielkość ziarna byłego austenitu w staliwie G20CrMo2-5 (L20HM). Badania przeprowadzono na próbkach pobranych z długotrwale eksploatowanego (około 160 000 godzin) kadłuba turbiny parowej. Opis wpływu parametrów austenityzowania na wielkość ziarna byłego austenitu dokonano za pomocą: średniej średnicy i średniego pola powierzchni ziaren. Przeprowadzone badania wykazały, że: rozkłady średnich średnic i średnich pól powierzchni ziaren mają charakter logarytmo-normalny na poziomie istotności OFO.01. Średnia średnica i średnie pole powierzchni ziaren w zakresie temperatury austenityzowania 895^985°C i dla czasów wygrzewania 3 i 5 godzin nie wykazują istotnych różnic. Znaczący rozrost ziarna obserwowano w temperaturze powyżej 955°C. Wykazano ponadto, że dla badanego staliwa optymalny zakres temperatur austenityzowania wynosi Ac3 + 30^60°C.