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SciVerse ScienceDirect Procedia
Engineering
Procedia Engineering 55 (2013) 433 - 437 ;
www.elsevier.com/locate/procedia
6th International Conference on Creep, Fatigue and Creep-Fatigue Interaction [CF-6]
Effects of Boron and Cerium on Creep Rupture Properties of Modified 9Cr-1Mo Steel and its Weld Joint
K. S. Chandravathia*, K. Lahaa, Norio Shinyab , M. D. Mathewa
aMetallurgy and Materials Group, Indira Gandhi Centre for Atomic Research,Kalpakkam- 603 102, India b Nano-Materials Group, National Institute for Materials Science,1-2-1, Sengen, Tsukuba, Ibaraki 305-0047, Japan
Abstract
Modified 9Cr-1Mo steel (9Cr-1Mo-0.2V-0.06Nb-0.1C-0.05N) was micro alloyed with boron («100 ppm) and cerium («80 ppm) to investigate its effect on creep rupture strength of the base steel and its weld joint. The microalloying was with the understanding that boron will increase the microstructural stability to increase the creep rupture strength and cerium will suppress the creep cavitation through its control over the soluble sulphur. Creep tests were carried out on the steels of T91, T91 micro-alloyed with boron, and T91 micro-alloyed with boron and cerium and its weld joints at 923 K over a stress range of 50 - 140 MPa. It was observed that the difference in creep rupture strength of the above base metals and its weld joints is marginally appreciable. Further, the steels were subjected to annealing at temperatures in and around the intercritical temperature range (1023 - 1273 K). Intercritical annealing reduced the hardness and tensile strength of the steels. Boron and cerium additions in the steel also resulted in marginal improvement on the hardness and tensile strength of the steels on intercritical annealing. It has been concluded that within the ASTM specification of the T/P 91 steel, the amount of nitrogen present concerned that the additions of 100 ppm boron and of 80 ppm cerium, and have no appreciable effect on creep rupture strength of the steel and its weld joint.
© 2013 The Authors.Publishedby Elsevier Ltd.
Selection and peer-review underresponsibility of the Indira Gandhi Centre for Atomic Research. Keywords: Modified 9Cr 1Mo steel; boron; creep; hardness
1. Introduction
Cr-Mo ferritic steels are widely used in power plants because of its excellent high temperature creep strength, resistance to stress corrosion cracking, low oxidation rate and good weldability. Power plant components are usually fabricated employing fusion-welding techniques. The weld joint has heterogeneous microstructures of the deposited weld metal, the HAZ and the base metal. The HAZ itself has complex microstructures, and also soft zone formation occurs in the intercritical-HAZ. This result in preferential accumulation of strain during deformation leading to premature failure of the weld joint in the intercritical-
^Corresponding Author: E-mail address: ksc@igcar.gov.in
ELSEVIER
1877-7058 © 2013 The Authors. Published by Elsevier Ltd.
Selection and peer-review under responsibility of the Indira Gandhi Centre for Atomic Research. doi: 10.1016/j.proeng.2013.03.276
HAZ at high temperatures and lower stress conditions and commonly known as Type IV cracking [1] which occurs at the outer edge of HAZ is considered as life-limiting factor of high-temperature welded components. Extensive studies on Type IV cracking behaviour have been reported by changing the welding parameters and alloying elements. Studies by Kondo et al [2] reported that Type IV cracking can be eliminated by alloying 9Cr steels with relatively high concentrations of boron and low concentrations of nitrogen. However, the understanding of mechanisms responsible for soft zone formation (Type IV) is not well understood so far. In this present investigation, effect of boron and cerium addition in modified 9Cr-1Mo steel on creep rupture behaviour has been studied.
2. Experimental Details
The steels investigated in this study were T91, T91 micro-alloyed with boron, and T91 micro-alloyed with boron and cerium. These materials are supplied by NIMS, Japan in 12mm thick plates. The above materials differ from the normal commercial variety by closely controlling the composition, lower residual element concentration, and lower inclusion content. The similar weld joints were fabricated by manual metal arc welding process employing a basic coated modified 9Cr-1Mo welding electrodes. The weld pads of the steels were subjected to a post weld heat treatment (PWHT) for 1 hour at 1033 K, and were subsequently X-ray radio graphed for their soundness. Creep tests were carried out on the base metals and their similar cross weld joints of the steels at 923K in the range 50 -140 MPa. The chemical compositions of the steels and deposited weld metals are shown in Table-1. Base steel samples with dimension of about 10 x 10 x 10mm were soaked at temperature in the range 973 -1273 K (below Ac! to above Ac3) at intervals of 25K for 5min each and subsequently quenched in oil. The quenched samples were tempered at 1033K for 1hr. Metallographic samples were prepared from the heat-treated samples. An immersion etching in villela's reagent (1% picric acid and 5% hydrochloric acid in ethanol) was found adequate to reveal the microstructures of the steels. Optical micrographic examinations were carried out by using "Leica Optical Microscope". Vickers's hardness measurements under load of 10kg were carried out on the quenched and tempered samples. Further, steel tensile blanks with dimensions of about 60 x 10 x 10mm were soaked at temperature in the range 973 -1273 K (below Ac! to above Ac3) at intervals of 100 K for 5min each and subsequently quenched in oil and then tempered at 1033 K/1hr. Cylindrical tensile specimens of 26mm gauge length and 4mm diameter were fabricated from these steel blanks with gauge length parallel to rolling direction of the plate. Tensile tests were carried out at a nominal strain rate of 3 x 10-4 s-1 at room temperature in a screw driven tensile testing machine equipped with a resistance heating furnace and digital data acquisition system.
Table.1. Chemical compositions of the steels and deposited weld metals.
Material Cr Mo Nb V Ni S C Si Mn N P B Ce Fe
T91 8.72 0.90 0.08 0.22 0.1 0.01 0.1 0.32 0.46 0.05 0.012 - - Bal
T91B 8.72 0.90 0.08 0.22 0.1 0.01 0.1 0.32 0.46 0.05 0.012 0.01 - Bal
T91BCe 8.72 0.90 0.08 0.22 0.1 0.01 0.1 0.32 0.46 0.05 0.012 0.01 0.008 Bal
Weld metal 8.63 1.02 0.06 0.2 0.63 0.008 0.098 0.035 0.58 0.006 0.011 - - -
3. Results and discussion
The microstructure of the T91, T91 micro-alloyed with boron, and T91 micro-alloyed with boron and cerium steels in the normalised and tempered condition was a typical tempered martensitic structure and are shown in Fig.1a, b and c. Hardness test has been carried out on T91, T91B, and T91BCe specimens soaked for 5 min at various temperatures in the range 973 -1273 K at the intervals of 25K. The variation of hardness for the as quenched steels with respect to soaking temperatures are compared for all the three steels and shown in Fig-2. Intercritical annealing heat treatment reduces the hardness of the steels. The same profile is being
followed by all the steels. The as quenched hardness increased further on soaking at temperature above Ac3 even though it attained 100% martensite at Ac3. With the increase in soaking temperature to above Ac3, the carbonitrides dissolve along with the austenitisation. This dissolution is greater with increase in soaking temperature. On cooling the austenite would lead to the formation of finer martensite owing to depression of Ms (martensite start) temperature caused by solute enrichment. This would lead to increase in hardness with increasing soaking temperature above Ac3. The effect of tempering on hardness of the quenched steel at temperature 1033K for 1hr is shown in Fig 3. Hardness of the quenched steel decreased with tempering. On tempering, a noticeable reduction in hardness of all the three steels were seen for soaking temperatures in the intercritical temperature range (Ac1-Ac3), resulting in a hardness trough whereas at temperatures below Ac1 a very marginal decrease in hardness was noticed. After Ac3 temperature the hardness increased. Boron and cerium additions in the steel resulted in no appreciable effect on the hardness of the steels on intercritical annealing.
■ : " v.-;«,-.,- „ - , • .
Fig. 1. Microstructures of (a) T91, (b) T91B, and (c) T91BCe steels.
Fig . 2. Variation of as-quenched hardness with soaking temperature.
Fig . 3. Variation in tempered hardness with soaking temperature.
Tensile tests at room temperature were also carried out on the isothermally heat treated at various temperatures in the range 973 -1273 K on T91, T91 micro-alloyed with boron, and T91 micro-alloyed with boron and cerium steels. The variation of the 0.2% offset yield strength and the ultimate tensile strength with soaking temperature are shown in Figs. 4 and 5. The strength was found to increase with soaking temperature above Ac3 at all test temperatures. The increase in tensile strength, as with increase in hardness, may be the attributed to fine martensitic structure with the smaller lath size and finer distribution of carbides (after tempering) developed in the steel with the increasing in soaking temperature above Ac3.
Fig .4. Variation of yield stress with soaking temperature at quenched condition.
Fig. 5. Variation of ultimate tensile stress with soaking temperature at tempered condition.
Creep tests were carried out on the steels and its weld joints at 923 K over a stress range of 80 - 140 MPa. Variations of the creep rupture life and steady state creep rate with applied stress for the base metals and weld joint are shown in Fig-6, 7 and 8. Weld joint of the steels possessed significantly lower creep rupture strength than the respective base metals. The failure occurred in the soft intercritical HAZ region of the joint. The failure was associated with preferential creep cavitation accompanied with localized creep deformation in the intercritical region of HAZ. There is no substantial difference in life was observed in T91, T91 micro-alloyed with boron, and T91 micro-alloyed with boron and cerium steels. No appreciable difference in type IV cracking behaviour of the steel on boron and cerium addition was observed and the joint of the steels possessed comparable creep rupture strength. The addition of boron has been reported [3] to improve the creep rupture strength of 9Cr ferritic steels. It reduces the rate of the Ostwald ripening of M23C6 carbides in the vicinity of prior-austenite grain boundaries (PAGBs) during exposure at elevated temperatures. Soluble boron contributes to the enrichment in M23C6 carbides, and soluble nitrogen causes the precipitation of nanosize MX carbonitrides. Both aids in strengthening by retarding the coarsening of precipitates which in turn improves the sub-boundary hardening which is the most important factor in long-term creep strengthening. But studies have shown that very judiciously calculated B and N have to be introduced as boron has a strong tendency to form BN. Sakuraya et al. [4] have shown a relationship between Boron and nitrogen concentrations and the size of boron nitride particles formed in various 9 to 12% Cr steels. The formation of large boron nitride particles offsets the above benefits of boron and nitrogen. In the present investigation the excess addition of nitrogen (500 ppm) might be the cause of the formation of large amount of boron nitrides which significantly decreases the boron concentration available for the microstructural stabilization and hence no beneficial improvement on creep strength was observed.
Addition of cerium along with presence of boron in austenitic stainless steel has been reported to suppress the creep cavitation through the segregation of film of boron on the cavity surface to increase the creep strength and ductility [5, 6]. Formation of boron nitride in the present alloys resulted in unavailability of boron in the matrix to suppress the creep cavitation and also to enhance the self healing effect. Hence, addition of cerium in the present alloy modified 9Cr 1Mo steel containing boron did not influence significantly on creep rupture properties.
Fig. 6. Variation of creep rupture life with applied stress for the T91, T91B and T91BCe base steels at 923K.
Fig. 7. Variation of creep rupture life with applied stress for the T91, T91B and T91BCe steels weld joints at 923K.
Applied stress. M Pa
Fig . 8. Variation of steady state creep rate with applied stress for the T91, T9Boron and T91 Boron Cerium base steels at 923K.
4. Conclusions
Following conclusions were drawn from the present investigation. The addition of 100 ppm boron and 80 ppm cerium in modified 9Cr -1Mo steel have not enhanced the rupture life and steady state creep rate significantly because of higher amount of (500 ppm) nitrogen presence. The hardness and tensile properties were also showed no significant variations. The amount of nitrogen presents in T91 steel concerned that the additions of 100 ppm boron and of 80 ppm cerium, and has no appreciable effect on creep rupture strength of the steel and its weld joint.
References
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[3] Fujio Abe,' Precipitate design for creep strengthening of 9% Cr tempered martensitic steel for ultra-supercritical power plants' Sci.Technol. Adv.Mater. 9(2008)013002(15pp)
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