C. S. SASMAL et. al.

1

EFFECT OF SIMULATED POST WELD HEAT-

TREATMENT ON THE MICROSTRUCTURE AND

MECHANICAL PROPERTIES OF IN-RAFM STEEL

C. S. SASMAL

Institute for Plasma Research

Gandhinagar, India

Email: chandra@ipr.res.in

Abstract

Indigenously developed India specific Reduced Activation Ferritic/Martensitic (IN-RAFM) steels are currently

considered as a structural material for the Indian Lead Lithium cooled Ceramic Breeder Test Blanket Module (IN-LLCB

TBM). Advanced ferritic/martensitic steels offer the opportunity for improvements in fusion reactor performance,

operational lifetime and reliability, superior neutron radiation damage resistance, higher thermodynamic efficiency and

reduced construction costs. Typically RAFM steels are normalized at high temperature i.e. 980oC for 30 minutes followed by

low temperature tempering for longer duration i.e. 760 oC for 90 minutes. The resulting microstructure determines the

mechanical properties of the steel. These microstructures are designed to produce an optimum combination of strength and

toughness at high temperature. However, situations may arise in practice, particularly during welding operations for

example, whereby the RAFM steel may receive an additional heat treatment which briefly exceeds the Ac1 and possibly the

Ac3 temperature before stabilizing at the tempering temperature. To restore the properties of the weld joints, post weld heat

treatment (PWHT) is applied to the steel at 750oC for 2 hours followed by furnace cooling. During PWHT, the base or

parent metal of the RAFM steel weld joints also undergo heat treatment process. In this present investigation, the

consequence of PWHT effect on base metal of IN-RAFM steel is studied. Simulated post weld heat treatments (SPWHT)

have been applied to IN-RAFM steel in a muffle furnace at 750 oC and 770 oC for 2 hours followed by cooling inside the

furnace. Hardness measurements were carried out on the heat treated sample and was found to be ~210 HVN which is

comparable with base metal hardness properties. Advanced electron microscopy has been carried out to investigate the effect

of the SPWHT excursions on subsequent microstructural evolution. Tensile tests have been carried out on SPWHT

specimens at various temperatures from room temperature to 600 oC. Tensile properties of SPWHT specimen at room

temperature is ~650 MPa and at 550 oC is ~320 MPa. Impact toughness upto -100 oC are also being evaluated in this present

investigation. The results discuss the effect of SPWHT on mechanical properties of RAFM steel during high temperature

service.

1. INTRODUCTION

Reduced activation ferritic martensitic (RAFM) steel is the candidate structural material for first wall and

tritium breeding blanket applications in DEMO reactor. RAFM steels are chosen for their good void swelling

resistance, thermo-physical and mechanical properties compared to austenitic steel [1]. Indigenously developed

India specific Reduced Activation Ferritic/Martensitic (IN-RAFM) steel is currently considered as a structural

material for the Indian Lead Lithium cooled Ceramic Breeder Test Blanket Module (IN-LLCB TBM) to be

tested in ITER[2]. Similarly, all partner countries are working toward the development of its own indigenous

reduced activation steel as their structural material [3], [4]. The chemical composition of 9Cr–1Mo–0.06Nb–

C. S. SASMAL et. al.

0.2V–0.05N has been modified by substitution of highly radioactive (induced) elements like Mo with W and Nb

with Ta to produce RAFM steel [5].

Due to complicated structure of TBM, joining and inspection technologies are very important for the

fabrication of TBM with numerous joint configurations with limited accessibility for welding. Due to this

reason, many researchers have addressed with weldability of IN-RAFM steel. Shaju et al. [6] studied the

properties in tungsten inert gas (TIG) welding of IN-RAFM steel. They compared the microstructures and

mechanical properties of the weld metal (WM) and heat-affected zone (HAZ) before and after post weld heat

treatment (PWHT). Vasantharaja and Vasudevan [7] attempted to develop welding materials for RAFM steel,

i.e. specially an activated flux for TIG welding. Srinivasan et al. [8] also developed filler wire for various

welding process of IN-RAFM steel, including the TIG, narrow gap TIG (NGTIG) and hybrid laser welding

processes. C. R. Das et. al. [9] optimized the welding parameters of laser and electron beam (EB) welding for

IN-RAFM steel. In addition, they established the optimum PWHT condition as 750 °C for 2 h. Thomas paul et.

al. [10] have studied the microstructure variation of dissimilar joints of SS 316L and IN-RAFM steel and

optimised the PWHT temperature as 750 °C for 1 h.

In this context, mechanical properties of the IN-RAFM steel base metal will be negated to a certain extent

after PWHT. In practice, in order to guarantee the performance of post weld heat treated steels, it is necessary to

estimate the performance of the as-received state steels by applying simulated post weld heat treatment

(SPWHT). Consequently, it is of great significance to elucidate the changes of the microstructure after SPWHT,

which dictate the ensuing properties of end products.

In this present paper, the validated PWHT conditions were carried out on base metals of IN-RAFM steel to

evaluate the mechanical properties and microstructural features. Also SPWHT were carried out at 770 °C for

1hr & 2hrs and 750 °C for 1hr and 2hrs followed by furnace cooling to evaluate the properties subjected to

overshoot temperature during tempering.

2. EXPERIMENTAL:

IN-RAFM steels procured from M/s MIDHANI have been used in the present study. The steel was used in

the standard heat treated condition namely normalizing at 980 oC/0.5 h followed by tempering at 760 oC/1.5 h.

Chemical composition of IN-RAFM steels is given in Table 1. The set of specimen blanks were cut and

subjected to SPWHT for 2hrs &1hr at 750 and 770 °C followed by furnace cooling. Similarly, few set of blanks

followed the same SPWHT heat treatment followed by air cooling. Then cuboid samples with dimensions 10

mm x 10 mm x 10 mm were cut from these blanks for microstructure characterization. Subsequently, these

samples were mechanically ground, polished, and chemically etched with vilellas reagent (100 ml ethanol, 5 ml

HCl and 1 g picric acid). Microstructures of as-received and SPWHT specimens were examined by optical

microscope and field-emission scanning electron microscope (FE-SEM). Vickers hardness test at 10 Kg load

have been carried out on heat treated cuboid samples.

Charpy Impact samples (10mm x 10mm x 55mm) were fabricated from these heat treated steel blanks.

The impact properties of all specimens (ASTM A370-11) were then obtained using a Charpy V-notch impact

test machine (ZWICK/ROELL, Germany) in the temperature range of -100 – +20 °C. To confirm the effect of

C. S. SASMAL et. al.

3

the microstructure on the failure mode during the impact test, the fractured surface after the impact test at +20

°C, -60 °C and −100 °C were observed by SEM.

Table 1: Chemical composition of IN-RAFM steel in wt.%.

Elements Cr C Mn V W Ta N

Composition (wt

%)

9.1 0.11 0.56 0.23 1.39 0.08 0.037

O P S B Ti Nb Mo Ni

< 0.0005 0.002 0.002 < 0.001 0.001 0.001 0.001 0.003

Cu Al Si Co As+Sn+Sb Fe

0.001 0.004 0.002 0.001 < 0.03 Balance

3. RESULTS and DISCUSSION:

3.1. Microstructure:

The IN-RAFM steels are generally used in normalized and tempered condition. Figure [1] shows the

microstructure of as-received IN-RAFM steel. The as-received steel had typical tempered martensitic structure

with complex microstructure. SEM micrograph of the as received metal reveals that precipitates developed

during heat treatment were decorated along the grain boundaries, inter lath and intra lath grain boundaries. The

average size of prior austenite grains (PAGS) was evaluated as 10 ± 2 µm for as-received steel. Microstructure

revealed that the coarser precipitates on grain boundaries were rich in chromium and tungsten (Fig. 1) and is

considered as M23C6 types based on previous studies [5]. The fine inter-granular precipitates are rich in V and

Ta and are considered as MX types of precipitates based on literature studies [5]. The average hardness of as-

received tempered martensitic IN-RAFM steel is around 215 HV10.

Figure 1: Microstructure of as-received IN-RAFM steel; a) SEM micrograph, b) Optical micrograph

Figure [2] shows the microstructure for SPWHT condition at 750 and 770 °C for 2hrs and 1hr followed by

furnace cooling. All micrographs show the temper martensitic structure. The PAG are measured in all heat

treated conditions and compared with as-received results as shown in figure [3]. Finer grains of approx. 8.5µm

are observed in 770 °C for 2 hours compared to other conditions. Subsequently Vickers hardness was measured

in all SPWHT conditions and found to be increased with coarser PAG at 770 °C/1hr. Figure [4] show the

a b

C. S. SASMAL et. al.

hardness in all SPWHT condition and N&T condition of as-received IN-RAFM steel. SPWHT at 770 °C/2hrs

exhibit lower hardness with coarser grain size compared to 770 °C/1hr SPWHT. This may be due to formation

of large no. of precipitate at 770 °C/1hr SPWHT compared to 770 °C/2hrs SPWHT. However coarse grains

were observed in 770°C/1hr SPWHT, but due to large no. of precipitate, hardness found to be increased

compared to 770 °C/2hrs. No. of precipitates are found in similar range for the specimen SPWHT at 750 °C for

2hrs and 1 hrs. So, PAG played a major role in determining the hardness of SPWHT specimens. SPWHT at 750

°C for 2 hrs shows coarse grain compared to SPWHT at 750 °C for 1hr specimen in fig [3]. Which exhibit

higher hardness in the specimen heat treated at 750 °C for 1hr compared to heat treated at 750 °C for 2hrs.

Measured precipitate no. density and prior austenite grain size for all SPWHT conditions are mentioned in table

2.

Figure 2: SEM micrographs of various SPWHT IN-RAFM steel at a) 770 °C/2hrs, b)770 °C/1hr, c)750 °C/1hr,

d)750 °C/2hrs.

Table 2: Precipitate no. density and grain size variation for all SPWHT condition

Sr.No SPWHT Conditions Grain Size Number density

1 As-received (N&T) 10 4.55090

2 750/1hr/FC 9 3.38689

3 750/2hr/FC 9.96 3.5132

4 770/1hr/FC 10.2 5.15182

5 770/2hr/FC 8.5 3.93881

b a

c d

C. S. SASMAL et. al.

5

Figure 3: Variation in grain size in all SPWHT condition of IN-RAFM steel

Figure 4: Variation in hardness in all SPWHT condition of IN-RAFM steel

3.2. Charpy Impact Properties:

Charpy V-notch impact tests were conducted for all specimens and Fig.[5] present the charpy impact energy

plotted as a function of the test temperature. The impact energy of all specimens decreased with decreasing

temperature and thus typical ductile-to-brittle transition occurred. The failed samples at three different test

temperatures were also shown along with the curve, which indicate the presence of ductile fracture on testing at

20 °C compared to those tested at −60 and −100 °C. The upper shelf energy (USE) of the steel observed over

the temperature range around 0 to 20 °C decreased by around 25 J on SPWHT at 770 °C for 2 hours. Below

around −40 °C the transition temperature regime was observed which was changed gradually from ductile to

brittle. The ductile to brittle transition temperature (DBTT) has been estimated on the basis of 68 J [11] criterion

incorporating in the Charpy energy versus temperature curve. Finer the PAG of IN-RAFM steel on SPWHT at

770 °C fr 2hrs increases the USE by 25joule at 20 °C. The lesser precipitate no. density also considered as one

of the reason in increasing USE of the SPWHT steel at 770 °C for 2hrs. Effect of microstructure on DBTT

behavior of 9Cr-1Mo has been extremely studied by Moitra et al. [12]. They concluded that refined

microstructure having fine grain size with small globular precipitate particle and soft matrix decrease the DBTT

and increase the USE.

C. S. SASMAL et. al.

Figure 5: Variation in impact toughness in all SPWHT condition of IN-RAFM steel

3.3. Fractography:

In order to substantiate the impact failure observation of the INRAFM steel on SPWHT impact failure

specimen, fractographic studies have been carried out using SEM. SEM fractographic study of the impact

specimens revealed three distinct zones as shear lip, fibrous dimple and cleavage in an order away from the

notch tip (Fig. 6(a,b,c)). The fractographs of the impact tested specimens indicates to the occurrence of ductile

fracture at the upper shelf region, Fig. 6(a), and the cleavage fracture at the lower shelf region, Fig. 6(c).

Extensive plastic deformation at the Charpy V-notch front occurs on impact loading. Under rapid loading during

impact, plastic deformation occurs as shear lip around 45° to the loading axis. At temperature below the upper

shelf, cleavage cracks propagate leading to brittle failure. At higher temperatures the ductile shear lip and

fibrous dimple zone dominate whereas as lower temperature due to severe restriction on plastic deformation as

many of the slip systems became inactive, cleavage failure dominates.

a b

C. S. SASMAL et. al.

7

Figure 6: SEM Fractography of fractured Impact specimen: a) at 20 °C, b)at -60 °C, c)at -100 °C

4. CONCLUSION:

Based on the studies of the effect of SPWHT on microstructure and mechanical behavior of

INRAFM steel and their correlation with microstructure, following conclusions have been made:

1. SPWHT increased the hardness of the steel in terms of coarsening of PAG and increased precipitate

no. density.

2. Significantly no variation in impact toughness has been observed in all SPWHT condition.

3. Fractography studies of the impact specimens reveal the fracture behaviour of steel upon SPWHT

condition. Impact fracture clearly shows the cleavage fracture at lower temperature indicates brittle

failure.

ACKNOWLEDGEMENTS

The authors wish to thanks to Chirag Rawat and Mohit Tiwari from IITRAM for their help during impact

testing, SEM and Optical microscope characterization.

REFERENCES

[1]. R. L. Klueh, and A. T. Nelson. "Ferritic/martensitic steels for next-generation reactors." Journal of Nuclear

Materials 371, no. 1-3 (2007): 37-52.

[2]. A. N. Mistry, , C. S. Sasmal, Shiju Sam, N. Singh, A. Prajapati, J. P. Chauhan, H. M. Tailor et al. "Status of

India-specific Reduced Activation Ferritic-Martensitic steel and fabrication technologies development for LLCB

TBM." Fusion Engineering and Design 125 (2017): 263-268.

[3]. B. Van Der Schaar, F. Tavassoli, C. Fazio, E. Rigal, E. Diegele, R. Lindau, et al., Fusion Engineering &

Design, 69 (2003), pp. 197-203.

[4]. H. Tanigawa, T. Hirose, K. Shiba, R. Kasada, E. Wakai, H. Serizawa, et al. Fusion Engineering &

Design, 83 (2008), pp. 1471-1476.

[5]. K. Laha, S. Saroja, A. Moitra, R. Sandhya, M.D. Mathew, T. Jayakumar, E. Rajendra Kumar, Journal of

Nuclear Materials, 439 (2013), pp. 41-50.

[6]. Shaju K. Albert, , Chitta Ranjan Das, Shiju Sam, P. Mastanaiah, M. Patel, A. K. Bhaduri, T. Jayakumar, C.

V. S. Murthy, and Rajendra Kumar. "Mechanical properties of similar and dissimilar weldments of RAFMS and

c

C. S. SASMAL et. al.

AISI 316L (N) SS prepared by electron beam welding process." Fusion Engineering and Design 89, no. 7-8

(2014): 1605-1610.

[7]. P. Vasantharaja, and M. Vasudevan. "Studies on A-TIG welding of low activation ferritic/martensitic

(LAFM) steel." Journal of Nuclear Materials 421, no. 1-3 (2012): 117-123.

[8]. G. Srinivasan, B. Arivazhagan, S. K. Albert, and A. K. Bhaduri. "Development of filler wires for welding of

reduced activation ferritic martenstic steel for India's test blanket module of ITER." Fusion Engineering and

Design 86, no. 4-5 (2011): 446-451.

[9]. C. R. Das, S. K. Albert, Shiju Sam, P. Mastanaiah, G. M. S. K. Chaitanya, A. K. Bhaduri, T. Jayakumar, C.

V. S. Murthy, and E. Rajendra Kumar. "Mechanical properties of 9Cr–1W reduced activation ferritic martensitic

steel weldment prepared by electron beam welding process." Fusion Engineering and Design 89, no. 11 (2014):

2672-2678.

[10]. V. Thomas Paul, S. Saroja, S. K. Albert, T. Jayakumar, and E. Rajendra Kumar. "Microstructural

characterization of weld joints of 9Cr reduced activation ferritic martensitic steel fabricated by different joining

methods." Materials Characterization 96 (2014): 213-224.

[11]. ASTM E-23-88, Standard methods for notch bar impact testing of metallic materials, Annu Book ASTM

Standard, 1990, 03.01, pp 197–212.

[12]. A. Moitra, P.R. Sreenivasan, P. Parameswaran, S.L. Mannan, Material Science and Technology, 18 (2002),

pp. 1195-1200