Ignacy WIERSZYŁŁOWSKI, Jarosław SAMOLCZYK Metal Forming Institute, Poznań
Sebastian WIECZOREK, Ewa ANDRZEJEWSKA Poznań University of Technology, Poznań
Waldemar NIEMCZYK
State College for Practical Education in Leszno, Leszno
The influence of deep cryogenic treatment
on transformations during tempering of quenched
D2 steel studies of XRD, structures, DSC, dilatometry,
hardness and impact energy
Wpływ niskotemperaturowej obróbki kriogenicznej na przemiany
podczas odpuszczania zahartowanej stali D2. Badania rentgenowskie,
struktury, DSC, dylatometryczne, twardości i energii łamania
AbstractThe study analyses the influence of cryogenic treatment on the resultant structures, the development of changes during tem-pering and the properties obtained. It was observed that cryo treatment affects the properties through: a significant reduction in retained austenite content, gradual reduction in value of martensite tetragonality and an increase in the number of carbon atoms participating in transformations during tempering. The consequence of the transformations taking place during cryo-genic treatment is precipitation of η-carbide instead of ε-carbide during low tempering. The number of η-carbides precipi-tated is higher than that of ε-carbide. This affects the functional characteristics of steel.
Streszczenie
Podczas badań analizowano wpływ obróbki kriogenicznej na uzyskane struktury, przemiany podczas odpuszczania i uzyski-wane właściwości. Stwierdzono, Ŝe obróbka kriogeniczna wpływa na właściwości wskutek znacznego zmniejszenia zawartości austenitu szczątkowego, stopniowego obniŜenia tetragonalności martenzytu i zwiększenia liczby atomów węgla biorących udział w przemianach podczas odpuszczania. W wyniku przemian zachodzących podczas obróbki kriogenicznej podczas od-puszczania wydziela się węglik η zamiast węglika ε. Liczba węglików η wydzielających się podczas odpuszczania jest większa
niŜ liczba wydzielających się podczas obróbki konwencjonalnej węglików ε . Zmiany te mają wpływ na właściwości uŜytkowe stali.
Key words: cryogenic processing, supersaturation, dilatometry, DSC, tempering
Słowa kluczowe: obróbka kriogeniczna , przesycanie, dylatometria, odpuszczanie
1. INTRODUCTION
The influence of cryogenic treatment on the transformations occurring during tempering of quenched tool steel and on the properties of the products made of it (mechanical proper-ties, wear resistance, life time) has been studied for years [1, 2, 4, 5, 6, 7, 8, 9]. Cryo treatment after quenching and tempering leads to signifi-cant increase in tool durability as a result of a reduction in retained austenite content and
precipitation, at low tempering temperatures, of η-carbides which are finer than ε-carbides and more evenly distributed in the matrix. The aim of this study has been to establish the influence of cryogenic treatment on transfor-mations occurring in cooling and tempering of D2 tool steel.
2. RESEARCH METHODOLOGY
Specimens of D2 steel, were subjected to heat treatment, a schematic detailed diagram of which can be seen in Fig.1.
-200 0 200 400 600 800 1 000 1 200 0 50 100 150 200 Time min T e m p e ra tu re o C THT LTT 1030 450 170 20 -196
Fig. 1. Schematic diagram of heat treatment of D2 steel specimens
Rys. 1. Schemat przedstawiający obróbkę cieplną próbek ze stali D2
The heat treated samples were subjected to analysis of microstructure, hardness and fracture toughness. Retained austenite content and the degree of tetragonality of martensite (c/a) were also determined by means of XRD analysis.
Comparative DSC studies were also car-ried out as well as dilatometric tests, during heating of traditionally processed samples and samples subjected to cryo treatment after quenching.
3. RESULTS
The structures obtained after quenching are presented in Fig. 2, where undissolved carbides of various sizes can be seen against the matrix.
The results of hardness and fracture tough-ness tests for quenched steel as well as quenched and cryo treated steel have been pre-sented in Fig. 3.
Results of X-ray analysis of retained aus-tenite content after quenching and tempering have been presented in Fig. 4, and changes in martensite tetragonality – in Fig. 5.
Fig. 2. Structure of quenched D2 steel samples. Visible differences in size of primary carbides
retained after austenitisation
Rys. 2. Struktura próbek stali D2 po hartowaniu, widoczne róŜnice wielkości węglików pierwotnych
pozostałych po austenityzowaniu
The retained austenite contents
1 6 11 16 21 26 0 200 400 600 Tempering temperature oC R e ta in e d a u s te n it e V o l% quenching in oil
quenching and cryo treatment
Fig. 3. Comparison of hardness and impact energy values resulting from tempering of traditionally
treated and cryo treated D2 steel samples Rys. 3. Porównanie wartości twardości i energii
łamania po odpuszczaniu próbek po obróbce kriogenicznej i konwencjonalnej
10 20 30 40 50 60 70 0 200 400 600
Tem pering tem perature oC
H a rd n e s s ( H R C ) a n d i m p a c t e n e rg y ( J ) v a lu e s Hardness of quenched nad cry o treated steel Hardness of quenched steel Toughness of quenched and cry o trated steel Toughness of quenched steel
Fig. 4. Comparison of hardness and impact energy values after tempering of conventionally processed
and cryo treated samples
Rys. 4. Porównanie wartości twardości i energii łamania po odpuszczaniu próbek po obróbce kriogenicznej
i konwencjonalnej
The influence of quenching condition and tempering temperature on tetragonality of
martensite D2 steel 1,01 1,015 1,02 1,025 1,03 1,035 1,04 1,045 0 100 200 300 400 500 Tempering temperature oC Te tr a gona li ty of m a rt e ns it e quenching and cryo treatment quenching
Fig. 5. Comparison of tetragonality changes during tempering of conventionally treated D2 steel samples
and cryo treated ones
Rys. 5. Porównanie zmian tetragonalności podczas odpuszczania próbek ze stali D2 po obróbce
konwencjonalnej i kriogenicznej
Dilatometric tests during steady cooling in lowered temperatures proved a second mar-tensite transformation (growth in length), visi-ble in Fig. 6.
Dilatometric analysis during tempering of quenched samples proves that the shrinkage accompanying precipitation in low tempera-tures (Fig. 7) is initiated at slightly lower tem-peratures (ca. 30 °C) in cryo treated samples than in traditionally treated samples, and is larger.
The results of those analyses have been presented in Fig. 8.
Changes of length during cryo treatment of quenched D2 steel -1,77 -1,72 -1,67 -1,62 -1,57 -1,52400 450 500 550 600 650 700
Time of cryo treatment in seconds
L e n g th
Fig. 6. The second martensite transformation in the dilatometric curve of quenched D2 steel
cryo treatment
Rys. 6. Druga przemiana martenzytyczna na wykresie dylatometrycznym chłodzenia zahartowanej stali D2
w temperaturach ujemnych
Fig. 7. Elongation of a quenched D2 steel sample during tempering and a diagram of shrinkage determination
Rys. 7. WydłuŜenie zahartowanej próbki ze stali D2 podczas odpuszczania i schemat określenia
zachodzącego wtedy skurczu
-2 0 2 4 6 8 10 0 100 200 300 400
Temperature during continuous heating o C
R e la ti v e c o n tr a c ti o n cryo treatment conventional treatment
Fig. 8. Comparison of changes in contraction value during tempering of conventionally quenched
and cryo treated D2 steel
Rys. 8. Porównanie zmian wartości skurczu podczas odpuszczania stali po kriogenicznej i konwencjonalnej
DSC test results for tempering of both types of specimens (traditionally treated and cryo treated) can be found in Fig. 9. Differ-ences between DSC curves are apparent for the two types of specimen.
NC11LV
Quenched from 1030 oC +heated 15oC/m in
0 1 2 3 4 5 6 7 0 100 200 300 400 500 Tem perature [ oC] H e a t fl o w [ m W ] NC11LV
Quenched from 1030oC + LN2 + Heated 15oC/min -2 0 2 4 6 8 10 12 14 16 18 0 100 200 300 400 500 Temperatura [oC] H e a t F lo w [ m W ]
Fig. 9. DSC diagrams – on the left: traditional treatment; on the right: cryo treatment
Rys. 9. Wykresy DSC nagrzewania zahartowanych stali – po lewej stronie po konwencjonalnej obróbce cieplnej,
po prawej po obróbce kriogenicznej
In the DSC graph for cryo treated speci-mens, much higher endothermic effects have been achieved. The visible steep high exother-mic peak is caused by retained austenite trans-formation, the maximum of the peak being at 380 °C. Retained austenite transformation in traditionally treated specimens begins at ca. 410 °C.
4. ANALYSIS AND DISCUSSION OF RESULTS
Structures obtained in quenching (Fig.1) suggest heterogeneity of the chemical constitu-tion of the matrix, because the amount and size of primary carbides retained after austenitisa-tion vary and may lead to macroscopic hetero-geneity of the matrix. Hardness and fracture toughness test results (Fig. 3) as well as re-tained austenite content (Fig. 4) do not exclude it. Martensite tetragonality tests (Fig. 5) indi-cate that tetragonality after cryo treatment is slightly lower than after traditional treatment and grows to the value obtained after tempering in 170°C for both variants of thermal treatment of steel. Martensite tetragonality tests were repeated on a higher number of specimens for both types of thermal treatment and a range of values was obtained, indicating that the lower level of martensite tetragonality may (for the steel under analysis) appear in a tradi-tionally treated specimen [publication in pro-gress].
The occurrence of two Ms temperatures during sub-zero treatment has been observed in numerous earlier publications of other au-thors [8,9]. A diagram of such a transformation can be seen in Fig. 10.
Fig. 10. A diagram of sample length changes when cooled to sub-zero temperatures.
Two Ms temperatures visible
Rys. 10. Wykres zmian długości próbki podczas hartowania do temperatur ujemnych.
The reason of Ms temperature lowering during martensite transformation may be:
− the existence of an austenite fraction of higher carbon and alloying element con-tent,
− austenite stabilisation through factors hin-dering its transformation into martensite, such as: plastic deformation of austenite or dislocation of carbon atoms in austenite so that the transformation of austenite into martensite requires cooling to lower tem-perature.
The influence of carbon content on Ms and Mf temperatures are shown in Fig. 11.
Fig. 11. The influence of carbon content on To, Ms and Mf temperatures (diagram). To is a temperature at which austenite and martensite are at equilibrium [3]
Rys. 11. Wpływ zmian zawartości węgla w stali na temperatury To, Ms i Mf stali podczas hartowania
(schemat). To jest temperaturą, w której austenit i martenzyt są w stanie termodynamicznej równowagi
In D2 steel, three different cases may occur: involving austenite fraction with a higher alloying element content, austenite fraction with higher carbon content, or both of these factors combined.
DSC and dilatometric tests of the tempe-ring processes dutempe-ring steady heating were per-formed in such a way that the specimens, after being cooled to liquid nitrogen temperature, were held in it for 24 hours, then brought gradually to room temperature, and only then records of changes in length (dilatometric tests) were made while DSC tests were performed during isochronal heating.
Results of both tests indicate clearly that changes in D2 steel after each of the thermal
treatments were initiated already at tempera-tures below 100°C. In cryo treated specimens peaks visible in DSC results were more appar-ent and bigger than in traditionally treated specimens. The changes affected martensite, whose amount was by 20% higher in cryo treated D2 steel. Shrinkage in dilatometric tests of cryo treated samples was, however, higher than it would have resulted from the higher martensite content.
Retained austenite transformation, during tempering of quenched steel in calorimetric tests or DTA, is defined as an apparent exo-thermic peak (see Fig. 12). The size of the area below the peak is directly proportionate to re-tained austenite content.
Fig. 12. DTA curves obtained in steady rate heating (tempering) of 100Cr6 steel samples quenched
from various temperatures [14]
Rys. 12. Wykresy DTA uzyskane podczas nagrzewania ze stałą szybkością (odpuszczania) zahartowanych
z róŜnych temperatur próbek ze stali 100Cr 6 When analysing the results, the influence of shrinkage the sample undergoes in the sub-zero treatment must be considered. The value of the shrinkage is three times as high as the increase in volume caused by martensite transformation of the 20% of austenite retained in D2 steel after quenching to room tempera-ture. Martensite, as presented in numerous pub-lications [3,15,16,17], is an unstable structure and that is why it tends to achieve a state close to equilibrium. Therefore, it may be expected that during long holding at -196°C a reduction in specific volume of martensite will occur as a result of shrinkage.
Haykawa and co-authors’ research [18] has proven that after cryogenic treatment at
-200 °C, the ageing processes begin already at ca. -120 °C and develop in two stages: the first one from ca. -120 °C to ca. -50 °C, and the second – from room temperature.
The first process is non-diffusional, tetra-gonality of martensite (c/a) lowers gradually as a result of which the martensite obtained has constant volume of the elementary cell, and the reaction is irreversible. The changes are shown in Fig. 13 [18].
Fig. 13. Martensite tetragonality changes of quenched and sub-zero treated samples of Fe 0.87%, C – 24% Ni alloy during temperature increase from -200 oC up to the
room temperature[18]
Rys. 13. Zmiany tetragonalności martenzytu zahartowanych i wymraŜanych próbek ze stopu Fe – 0,87% C -24% Ni i podczas wzrostu temperatury
od -200 oC do temperatury pokojowej
Starting from room temperature a reaction involving carbon diffusion begins, during which c/a still diminishes, cells with regular and tetragonal network coexist, the volume of matrix cells diminishes and, simultaneously, doublets of tetragonal peaks appear characte-rised by abnormally high intensity, which may indicate the emergence of clusters [18].
With further increase in temperature pre-cipitations of η-carbide and not ε carbide ap-pear. The difference between these carbides lies in the arrangement of carbon atoms. In η carbide carbon atoms build a sub-network, filling half of the octahedral gaps among iron atoms, which causes formation of additional reflexes, absent from ε-carbide. η-carbide is identifiable during tempering up to 200 °C. As follows from dilatometric tests performed in Miller & Breyer [19] on steel containing
0.47%C and 0.7%Mn, together with plastic deformation of quenched samples (within 1.5-5.5%), the value of shrinkage caused by tempering in steady heating diminished. The authors explain this with carbon atoms being bound by vacancies generated during plastic forming – this is so powerful that car-bon atoms did not participate in tempering processes up to the temperature of ca. 380 °C.
The increase of the value of shrinkage dur-ing temperdur-ing of cryogenic treated specimens may be then caused by: increase of martensite content in the specimen (from ca. 72% to 94%) and the ejection of C atoms from their positions prior to cryogenic treatment as a result of mar-tensite cells shrinkage.
The content of carbon equivalent to the number of those atoms can be estimated at ca. 0.2%, which may be calculated into the value of relative shrinkage of 7.2%, that is, close to the shrinkage obtained in dilatome-tric tests of D2 (Fig. 8). DSC tests results (Fig. 9) confirm the dilatometric tests. In the traditionally treated specimen the first exo-thermic effect, at ca. 70°C, is very blurred, and the second, with the maximum value at ca. 230 °C, very small. The peak appearing at in the range of 300-400 °C precedes the peak caused by retained austenite transformation (see Fig. 13). In the cryo treated specimen the maximum of the first peak appears at ca. 80 °C, the peak is evident and indicates that it is re-lated to one precipitation process. In the tem-perature range of 100—300 °C a peak appears with the maximum at ca. 200 °C. It is much higher than for the traditionally processed specimen, and it cannot be excluded that two precipitation processes overlap here. The next peak caused by retained austenite transforma-tion at ca. 380 °C is very narrow and high, which proves very dynamic development of the change. Such a change may follow the marten-sitic mechanism.
From the comparison of retained austenite transformation in traditionally treated and cry-ogenic treated samples it can be concluded that the transformation in the cryo treated sample is initiated at lower temperatures than in the traditionally treated one. The cause of the acceleration of the change may be: faster and more intensive development of the
pro-cesses preceding the retained austenite trans-formation in this sample and the small amount of retained austenite. The change happens very quickly, which is indicated by the martensite mechanism of the change [1,3,12].
5. CONCLUSION
On the basis of the research conducted, literature data and discussion of the results ob-tained, the following conclusions can be made: − the cause of precipitation of significant
amounts of η carbide during tempering, which improves numerous functional quali-ties, are changes in structure taking place in martensite during cryogenic treatment, − the changes are caused by the uniform
shrinkage of samples in cryogenic treatment.
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