Microstructure and Precipitation Behavior in Heat Affected Zone of Nitrogen-Enhanced Microalloyed Steel Containing V and Ti

Proceedings of Sino-Swedish Structural Materials Symposium 2007

Microstructure and Precipitation Behavior in Heat Affected Zone of Nitrogen-Enhanced Microalloyed Steel Containing V and Ti FANG Fang, YONG Qi-long, YANG Cai-fu, SU Hang (Center Iron 8z Steel Research Institute, Beijing 100081,China)

Abstract:

Three

steels:O.O5%C-O. 1%V-0.0 1%N

(steel

V-LN),

0.05QC-0.1 %V-0.02%N

(steel

V-HN),

0.05%C-O. 1 %V-0.02%N-0.01 %Ti (steel V-HN-Ti), all essentially vanadium microalloyed steels were subjected to

simulating the microstructure of a coarse grained heat affected zone (CGHAZ). This involved reheating to 1350%, rapid cooling to room temperature, including four cooling rates of tw5 equals to 7 5 ,20s. 4Os, 100s. as the relationship of heat input and tw5 was calculated by Quiksim software. The microstructure and precipitation of vanadium and titanium carbon nitrides were studied in this paper. The results showed that the microstructure consists of granular bainite and some side plate ferrite in the grain boundary when the steels were produced with the highest heat input. As the heat input decreased, there appeared numerous polygonal ferrite and grain boundary ferrite, and the size apparently grows up. For the limitation

of cooling time, vanadium carbon nitrides can’t precipitate sufficiently, but as titanium was contained, the unmelted or precipitated T f l on cooling absorbed some fraction of nitrogen in the matrix and make more precipitate positions for the round shaped V(C.N), so many useful round particles can be seen in titanium-contained steel, most of them were around TiN.

Key words: Nitrogen; HAZ;microstructure and properties; weld input; toughness

1. Introduction

contain high proportions of femte side-plates and bainite. Vanadium gives grain refinement and precipitation strengthening to HSLA steels. The effect of vanadium on the CGHAZ microstructure is quite different from that of niobium. Vanadium has a beneficial effect on the toughness of the CGHAZ, because it reduces the bainitic colony size and promotes intragranular nucleation of acicular ferriteL3’. Some papers have reported that nitrogen has a beneficial effect with proper heat input, but the

Femtic steels with high nitrogen contents (0.007%-0.02%) are often not utilized because of apparently harmful effects of nitrogen on both the base metal and weld area. Vanadium additions to high nitrogen steels offer distinct advantages in utilizing two mechanisms to obtain increased strengthening: grain refinement and precipitation hardening. The precipitation of V(C,N) is also effective in removing N from solid solution in ferrite and making the steel non-aging. In addition, with proper control over heat input and weld metal composition, it is possible to maintain good mechanical properties in the weld as we11”’21. During welding, the CGHAZ experiences peak temperatures up to the melting point, followed by rapid cooling. The high temperatures can lead to significant austenite grain coarsening and the combination of a coarse austenite grain size and rapid cooling promotes brittle microstructures, which

contents of nitrogen were often lower than 0.013% with relatively higher carbon content(>O.1%)[4951. However, the role of vanadium and nitrogen content on the microstructure and toughness of the CGHAZ and on the precipitation stability is not fully understood. Moreover, if a proper welding property can be obtained by reducing the carbon content to as low as 0.05%, and the compensation of strength by increasing nitrogen content to the level of 0.02%,we may achieve satisfied 249

Proceedings of Sino-Swedish Structural Materials Symposium 2007

HAZ microstructure and precipitates.

relationships of heat input energy and tw5 were calculated by Quiksim software from DSI. The peak temperature was 1350°C and 1s holding time.

The objective of this study was to assess the influence of vanadium and nitrogen on the microstructu~ and precipitates of the simulated CGHAZ in low carbon (0.05%) femte and perlite steels.

2.2 Metallographic and precipitation analysis The metallographic specimens were metallographically prepared by standard techniques, the microstructure of the simulated HAZ were

2. Experimental Procedure Alloy Compositions and Mechanical Properties The steels used in the experiment were manufactured in a high frequency induction vacuum

examined by optical microscopy and a JEOL scanning electron microscope. Prior austenite grain size in the CGHAZ was measured by a linear intercept method

melting furnace, cast to give 40kg ingots and forged to 13 mm thick plate. The chemical compositions of the steels tested are shown in Table 1. 2,lWeMing Thermal cycle

and a minimum 200 trains were counted for each specimen. The morphological, structure and chemical characterization of particles were achieved by means of a JEOL-JEM-2010 high resolution transmission

A welding thermal simulation experiment was

electron microscope.

3 Results and Discussion

used to generate a relatively large area of CGHAZ on Gleeble 3500, with the test blanks of 10.5*10.5*90 mm size for thermal simulation. The heat input was from 15 to 54 W/cm (tw5=7.5s, 20s, 40s and 100s) to simulate low to high heat input welding procedure, where the

3.1 Prior austenite grain size The microstructure of parent steel before welding thermal cycling process consists of perlite and femte is shown in Fig.1 (a). All investigated steels have the

Table 1. Chemical compositions of the experimental steel (mass %) code

C

Mn

Si

S

P

v

Ti

N

V-LN

0.051

1.5

0.4

0.008

0.0078

0.082

d.0005

0.009 1

V-HN

0.049

1.57

0.39

0.006

0.0064

0.092

<0.0005

0.018

V-HN-Ti

0.04

1.6

0.37

0.007

0.0099

0.09

0.0098

0.016

same microstructure and the volume fraction of femte is more than 80%, which is very beneficial to the toughness of the parent steel. The prior CGHAZ austenitegrain size of the V-LN,V-HN,V-HN-Ti steel is about 5 2 . 5 2 8 ~ 138.464p, . 78.186pm respectively, which is shown in Fig.1 (b). It can be seen that the addition of nitrogen has no effect in retarding the growth of austenite when the investigated specimens were reheated to 1350"C, but even worse when compared with low nitrogen steel. It can be reasoned that at the high peak temperature up to melting points, vanadium and nitrogen were all solved, and the pinning effect of particles faded away. The refinement of

From the research of Hamada16', the variation in the relative amounts of microstructural constituents accounts for these differences. In the other condition, nitrogen becomes an alloying element when combined with titanium, as TiN is easy to precipitate at high temperature, then the growth of high temperature austenite grain can be retarded, and good properties, such as ITT,CTOD, also can be obtained in the following process. . 3.2 Microstructureof simulated CGHAZ The parent microstructures are significantly changed after welding. The entirely perlite and femte are substituted by numerous femte and bainite

austenite grain and precipitation hardening effect only can be achieved by the precipitation of vanadium with carbon and nitrogen to form vanadium carbon nitride.

structures. The microstructure of the investigated three steels has some similarity as it is shown in Fig.2. When 250

Proceedings of Sino-Swedish Structural Materials Symposium 2007

Fig.1 Microstructure and grain size of parent steels: (a) microstructure of parent steel; (b) prior austenite grain size of investigated steels.

(a) tw5=7.5s;(b) tw5=20s;(c) tW5=4Os

Fig. 2 Microstructure of CGHAZ after echted by 4% natal (0.1V-0.016N-O.O1’l3 steel)

cooling rate is high, there is no grain boundary ferrite could be found in the matallographical paragraphs. The black field looks like martensite lathes which can be affirmed as granular bainite in larger magnification. There are also differences in the grain type and size. Though it’s easy to measure prior austenite grain size, it’s not so convenient to make quantitativeanalyze of bainite or ferrite grain size as several types of phase always mix with each other at the four cooling processes. But it’s obvious to observe that it’s easier to form large size granular bainite and polygonal ferrite for high nitrogen content steel. After adding titanium to the high nitrogen content steel, the grain size significantly reduced, but the content of granular bainite doesn’t reduce clearly. It can be explained that, the CGHAZ area experienced very high temperature heating-up, the vanadium carbon nitride wholly solved,

the solved vanadium acted as solution strengthening element just adds up ?he strength of HAZ, but have no contribution on keeping grains from growing up. At the same time, though the rate of V:N is higher than the stoichiometric, which are 9.1 and 5.1 for the two titanium-free steels respectively, vanadium can’t precipitate with nitrogen completely, so the rest nitrogen remained in the matrix and have no precipitation strengthening effect. When it was cooled to the following transformations, as the temperature goes down, the super-cooled austenite will transform to middle and low temperature microstructures- low carbon bainite and femte. During this process, vanadium carbon nitrides will precipitate in the bainite or ferrite with small grain size of 5-30nm. In the condition of high nitrogen and high cooling rate, vanadium was 25 1

Proceedings of Sino-Swedish Structural Materials Symposium 2007

combined with nitrogen first to form VN,it’s beneficial for the rest vanadium carbon nitrides to precipitate, but it’s easier to form the microstructure of granular bainite and martensite-austenite island in HAZ,which cause low toughness in the CGHAZ.

3.3 Precipitation in the CGHAZ When the welding thermal cycling is performed, the heat affected zone showed precipitates so few and single, it’s not easy to find mass distribution of carbon nitrides. Fig. 3 shows the precipitation of big and small size particles precipitated in austenite and femte.

Fig. 3 SEM micrographs showing few distribution

of V(C,N)and TiN precipitates

Examination of HAZ samples from vanadium and titanium microalloyed steels using SEM revealed quite different precipitate with as-received results. The vanadium and titanium carbon nitrides can be easily found in the grain or along the grain boundary in as-received steels[71.But in this experiment in CGHAZ, even a small number of precipitates can be hard to find out, where the reaSon was explained in the front part of this paper. There are also complex shape nanoparticles of V(CN) adherent around TiN in Femte, the diameters of the particles vary between 5-3Onm. The particle size larger than 20nm can be considered that they precipitated in austenite and very few. For the limitation of following cooling rate, V(C,N) have no time to precipitate in ferrite, so we can’t find many particles in the first two steels. Though V-HN steel contains more nitrogen and was considered easy to have more precipitates, it’s not observed in this experiment. So the high content of nitrogen has no use in precipitation and may even make worse results as the influence of free nitrogen. The wholly solved temperature of TiN is very high, which is 1503’C calculated by Thermo-calc. So after welding process, TiN may not be solved and some can be retained at high temperature, so the rectangular holes in SEM images can be considered as TiN. As an incoherent second-phase, the binding force of TIN and matrix 15 weak and easy to apart from matrix after \tre\\. \o it is defined as decohesion second-phase18’ Thanh:, for the unmelted or precipitated Ti,which absorbed some fraction of nitrogen in the matrix and make more precipitate positions for the round shaped

V(C,N), so many useful round particles can be seen in titanium-contained steel, most of them were around TiN.

4 Conclusions The microstructure and Precipitation behavior in Heat Affected Zone of Nitrogen-enhanced Microalloyed Steel in three microalIoyed low carbon, V-low N, V-high N and V-high N-Ti steels, produced by simulating coarse-grained heat affected zone process have been studied. It has been demonstrated that: ( I ) When cooling time t8/5 equals to 7 5 , the microstructure consists of granular bainite and some side plate femte in the grain boundary. As the increasing t ~ 5 ,there appeared numerous polygonal ferrite and grain boundary ferrite, and the size apparently grows up. (2) In the condition of high nitrogen contained, the effect of free nitrogen and high cooling rate increased, the vanadium carbon nitrides couldn’t precipitate completely, so more free nitrogen remained, which makes very few precipitates in the CGHAZ and large grain size. (3) For the limitation of cooling time, vanadium carbon nitrides can’t precipitate sufficiently, but in the titanium contained condition, the unmelted or precipitated TiN, they absorbed some fraction of nitrogen in the matrix and make more precipitate positions for the round shaped V(C,N), so many useful round particles can be seen in titanium-contained steel, most of them were around TiN. 252

Proceedings of Sino-Swedish Structural Materials Symposium 2007 Steels [J]. ISIJ International, 2001,41( 1):46-55. [5] F C Liao, S Liu, D L Olson. Weldability of

References:

Nitrogen-Enhanced HSLA Steels [C]. The American

[ I ] J N DuPont and A R Marder. The Influence of Nitrogen on of

Society of Mechanical Engineers.Proceedings of the 12n,

Microalloyed Steels. A Literature Review. Lehigh

International Conference on Offshore Mechanics and

University. unpublished research, 2001.

Arctic Engineering. 1993:231-243.

the

Microstructure

and

Mechanical

Properties

[2] N E Hannerz and B M Jonsson-Holmquist. Influence of

[6] M Hamada, Y Fukada and Y Komizo. Microstructure and

Vanadium on the Heat-Affected-Zone Properties of Mild

precipitation behavior in heat affected zone cf C-Mn

Steel [J]. Metal Science,1974,8:228-234.

microalloyed steel containing Nb, V and Ti [J]. ISU International. 1995,35(10):1196-1202.

[3] P H M.Hart and P S Mitchell. The effect of vanadium on the

[7] K A El-Fawakhry, M F Mekkawy and M L Mishreky.

toughness of welds in structural and pipeline steels. Weld research supplement, unpublished research, 1995.

Characterization of precipitates in vanadium and titanium

[4] Y LI, D N Crowther, M J W Green et al. The effect of Vanadium

and

Niobium

on

the

Properties

microalloyed

steels

[J].

ISU

International,

1991,3l(9): 1020- 1025.

and

[8] Y Q Long. Second-phases in Steel[M]. Beijing:

Microstructure of the Intercritically Reheated Coarse Grained Heat Affected Zone in Low Carbon Microalloyed

Metallurgical industry press,2006.

253