Microstructure and Hardness of T250 Maraging Steel in Heat Affected Zone

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-;;" ScienceDirect JOURNAL OF IRON AND STEEL RESEARCH, INTERNXI10NAL. 2009, 16(1): 87-91

Microstructure and Hardness of T250 Maraging Steel in Heat Affected Zone MO De-feng' ,

HU Zheng-fei! ,

CHEN Shu-juan' ,

WANG Chun-xu'' ,

HE Guo-qiu'

O. School of Material Science and Engineering, Tongji University, Shanghai 200092, China; 2. Central Iron and Steel Research Institute, Beijing 100081, China) Abstract, Electron-beam (EB) welding was used in T250 maraging steel, microstructures of both base material and heat affected zone (HAZ) were investigated by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) , and microhardness was tested. The results showed that during EB welding, the HAZ of T250 maraging steel exhibited a continuous gradient structure. The microstructure of the entire HAZ, from fusion line, could be divided into four zones: fusion zone, overheated zone, transition zone, and hardened zone. The microhardness showed a distinct regularity in each area. The softest region was the fusion zone, whereas the hardest was the hardened zone. In the overheated zone, the hardness increased as the grain size decreased. Furthermore, in the transition zone, the hardness level dropped noticeably. The peak temperature during the thermal cycle had a great influence on the formation of reverted austenite and dissolution of the precipitated particles, which contributed a lot to the microstructure and hardness of this material. Key words: T250 maraging steel; electron-beam weld; microstructure; microhardness

18 Ni Co-free maraging steels are a class of ultra high-strength steels characterized by very little carbon content and strengthened by the precipitation hardening mechanism during the aging time[l.Z]. They are widely used in critical applications such as rocket motor cases, cryogenic missiles, and landing gears[3.4]. For many of these applications, welding is one of the key directions to develop maraging steels. The microstructural changes in the weld fusion zone have been studied by many researchers[5.6], but less attention has been paid to the heat affected zone (HAZ) in maraging steels. Different areas in HAZ endure various peak temperatures during the welding process, which results in a continuous gradient structure. The transformation between austenite and martensite plays an important role in microstructural evolution. The reversion to austenite is partially responsible for softening and it has been suggested that it may be significant for fatigue and stress corrosion cracking[5.7]. Electron-beam (EB) welding is a process that

produces coalescence of metals with extremely high beam power density (about 10 5 - 10 7 W/cm 2 ) and has tremendous penetrating characteristics, with lower heat input, higher velocity, and smaller HAZ[8.9]. It is suitable for low-carbon steels such as maraging steels[lO]. However, the mechanical properties in the HAZ, especially in the overheated zone, usually behave poorly, and it is necessary to thoroughly understand the microstructures and performance of rnaraging steel in these areas. In the current study, T250 maraging steels are vacuum induction melted and consumable electrode remelted, the microstructure and hardness in the HAZ are investigated, and the relationship between the two is discussed.

1

Experimental

The examined T250 maraging steel in the form of a 1. 5 mm thick sheet was received in a solutionannealed condition. Welding was carried out under EB with a current of 28-32 rnA and travel speed of

Foundation Item, Item Sponsored by National Natural Science Foundation of China (50771073) Blography:MO De-feng(l982-), Male, Doctors E-mail: [email protected]; Revised Date: May 21. 2008

H 20, 8 g (N0 2 ) zC 6H 20H (Picric acid), 5 mL H 2S04 , 105 mL H 2 02 , and four drops of HF. Thin foils for TEM were prepared from the base materials and HAl, by grinding and electropolishing in a twin-jet apparatus, using a solution containing 20 % perchloric acid and 80 % methanol, at the temperature of - 40 ·C. Vickers hardness was measured for

400 - 1 000 mm/min, and no filler material was used. The chemical composition of the materials is given in Table l. Optical microscopy (OM) and SEM were employed to observe the continuous gradient structure and the grain size of the HAl. Before observation, the specimens were etched with a mixture of 100 mL Table 1

(mass percent.

Chemical composition of T250 maraging steel

%)

Element

Ni

Mo

Ti

C

Al

Co

Mn

Fe

Composition

18

3.0

1.4

<0.01

O. 1

0.5

0.1

Balance

20 s with the load of 2. 94 N.

2

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Results and Discussion

2. 1

Microstructure of base material The microstructure of the base material is shown in Fig. 1 (a). The grain size is about 10-15 11m and some randomly dispersed precipitates are discovered in the grains. Massive martensite blocks are seen to consist of bundles of parallel, heavily dislocated laths in the TEM, as shown in Fig. 1 (b). 2. 2

Microstructure of HAZ Fig. 2 (a) shows the whole appearance of the HAl. As the distance from the fusion line grows farther, the metal undergoes distinct peak temperatures during the thermal cycle in EB welding, which results in a continuous gradient structure. As the distance from fusion line changes, the whole HAl can be divided into four zones, fusion zone [Fig. 2 (b) J, overheated zone [Fig. 2 (c) J, transition zone [Fig. 2 (d) J, and hardened zone [Fig. 2 (e)]. The magnification microstructures of the area marked with 1, 2, 3, and 4 are shown in Fig. 2 (a), respectively. From Fig. 2 (b), a fusion line can be clearly ob-

Fig. 1

served. The top left side is the weld seam, and on the opposite side is the HAl. Welding microstructures show typical dendritic morphology, because the extremely high beam power density results in enormous gradients of temperature and provide an advantageous condition for the growth of dendritic crystals. In the region adj oining the weld metal, the peak temperature during thermal cycle is just below melting point and the metals here experience a short time of annealing solution treatment. The planar crystal strip that forms between the surface layer and the base metal promotes epitaxial growth from the partial fusion zone of the base metals. The main reason can be given as follows: there are little particles for nucleation during the condition of enormous gradients of temperature in the process of welding; and the crystals probably grow up along the edge of molten pool. Fig. 2 (c) shows the grain size in the overheated zone O. 5 mm away from the fusion line. The grain size decreases as the distance from the fusion line increases. It clearly shows that at high thermal cyclic peak temperature, the grain gets recrystallized and grows obviously. The grain size near the fusion line in the HAl is about 45 11m compared to 20 11m at a

Microstructures of the base material taken by OM (a) and TEM (b)

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Microstructure and Hardness of T250 Maraging Steel in Heat Affected Zone

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(a) Whole appearance of the HAZ, (b) Magnified microstructure of fusion zone, (c) Magnified microstructure of overheated zone; (d) Magnified microstructure of transition zone; (e) Magnified microstructure of hardened zone

Fig. 2

Microstructures of HAZ taken by OM

distance of O. 5 mm from the fusion line. The grain boundaries in this area exhibit grooving and the grooves are easily seen in Fig. 2 (b). It is believed that the presence of inclusions in maraging steel weldments liquate locally in the impurity-rich regions and then the grain boundary is markedly attacked by the etchant-'". Fig. 2 (d) shows the microstructure of the transition zone, which is deeply etched in the same etchant condition and exhibits a two-phase structure through the OM. It is about O. 2 mm wide and 2. 4 mm away from the fusion line. This area experiences a peak temperature in the range A c3 to A cl , some reverted austenite will therefore form in this region in the martensitic matrix and remain thus at room temperature. Next to it is the hardened zone [Fig. 2 (e) J, which shows the lath structure that contains a lot of

dispersed precipitate inside. TEM has been used to examine the microstructure of the transition zone. Fig. 3 (a) and (b) are bright field (B. F. ) and dark field (D. F. ) micrographs, respectively, showing the presence of a plate of the second phase, which has been identified as austenite precipitated on the lath boundaries. Many researches have discovered that the reversion to austenite starts at temperatures as low as 723 K[7]. Fig. 3 (d) is the schematic representation of the SAD pattern [Fig. 3 (c) J indicating the orientation relationship between martensite and austenite, in accordance with the Nishiyama-Wassermann relationship, which is shown as follows:

[001J'//[011]1 [110J.//[111]7 The grain boundaries were investigated using a

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(a) B. F. micrograph; (c) [113J, SADP;

Fig. 3

(b) D. F. micrograph taken from (220)y; (d) Schematic representation of the SADP

TEM investigation in the transition zone showing reverted austenite along the lath boundaries

SEM with high magnification, and Fig. 4 (a), (b), and (c) show the grain boundary microstructures in the overheated zone, transition zone, and hardened zone, respectively. From direct observation, the grain size in the overheated zone is remarkably bigger than that in any other region, which corresponds with the results of OM. Furthermore, with the same etchant condition, Fig. 4 (a) shows a clear grain boundary microstructure, whereas quite a significant amount of reverted austenite distributes along the grain and lath boundaries in Fig. 3 (b). The austenite exhibits a granulated morphology in the transition zone instead of the plate-like morphology, which has been reported previously. This is mainly due to the distinct etchants used[ll]. Moreover, in the hardened zone, the peak temperature during a thermal cycle never reaches Ad' and a little martensite can revert to austenite.

2. 3

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Microhardness in HAZ Fig. 5 shows the results of the microhardness examination in HAZ. It can be found that in the overheated zone, the hardness is much lower than that of

base material. The main reasons are that the high heating temperature of the overheated zone first austenitizes this zone completely, and the majority of the second phase particles dissolve. This is followed by the austenitic grains seriously growing-up, and after the transformation to martensite, the effects caused by the precipitation hardening mechanism are eliminated entirely. The hardness increases as the distance from the fusion line increases and the grain size decreases, which is in accordance with the HallPetch relationship. In the hardened zone, the hardness reaches the highest at about HV 630. This is because the highest temperature during the thermal cycle is just below Ad' reverted austenite cannot be formed, and the grain size does not grow bigger, but keeps fine. Besides, this area experiences a short time of aging treatment during EB welding, which results in high microhardness. As for this steel, the transition zone is a very important area, and the valley of hardness can easily be seen from the curve in Fig. 5. According to welding metallurgy[12], in this region, the microstructure is austenitized and recrystallized partially, and the density of

Microstructure and Hardness of T250 Maraging Steel in Heat Affected Zone

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(a) Overheated zone,

Fig. 4

(b) Transition zone,

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(c) Hardened zone

Microstructures of HAZ taken by SEM showing grain boundaries

and hardness. References: [lJ

400 300'LL.._........_~ _ _..L....L..-........_---oJ'--~ o 0.8 1.6 2.4 3.2 4.0 4.8 Distance from fusion linelmm

[ZJ

Fig. 5

[3J

Distribution of microhardness in HAZ

dislocation from the low temperature transformation decreases. The reverted austenite, which remains to room temperature, is softer than the martensite, resulting in low hardness. What is more, precipitating particles such as Ni, Ti and Ni, Mo segregate and accumulate, which also weaken their pinning effect on dislocations.

3

Conclusions

During EB welding, the HAZ of T250 maraging steel exhibits a continuous gradient structure. The microstructure features in the whole HAZ can be divided into four zones from the fusion line: the fusion zone, overheated zone, transition zone, and hardened zone. Each area is mainly composed of martensite, but shows distinct hardness. The softest region is the fusion zone with a microhardness value of about HV 330, whereas the highest is in the hardened zone, HV 630. In the overheated zone, the hardness increases as the distance from fusion line increases and the grain size decreases. The transition zone is a very important area, and the hardness level in this region decreases obviously. The peak temperature during a thermal cycle has a great influence on the formation of reverted austenite and dissolution of precipitated particles, which contributes a lot to the microstructure

[4J

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[5J

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