effect of thermomechanical treatment on...

METAL 2009 19. – 21. 5. 2009, Hradec nad Moravicí

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EFFECT OF THERMOMECHANICAL TREATMENT ON MECHANICAL PROPERTIES AND MICROSTRUCTURE EVOLUTION OF

MOLYBDENUM HSLA STEELS

M. HAKEM, H. BELHANNICHE, D. ALLOU.

Centre de Recherche Scientifique et Technique en Soudage et Contrôle. Division de Mécanique et Métallurgie

BP 64, Route de Dely Ibrahim, Chéraga, Algiers, ALGERIA [email protected]

Tel/Fax: + (213) 21 36 18 50 Abstract The current obligation, for certain technological applications, to acquisition of a high level of yield strength with a sufficient toughness and good weldability, also of a high hardenability and economic profits on finished products, rolled or forged, is the principal objective of the tendency towards the use of HSLA steels. The design of these alloys is walked on towards structures where several components of transformation coexist conferring on the latter an acquisition of a good properties. The objective to develop an HSLA steels with Molybdenum of controlled forging is to obtain a good technological and mechanical properties of hot formed parts while eliminating the need for the heat treatments. For this, a specific requirement to the manufacturing process is lead to select the thermomechanical controlled treatments to be the ideal synonym of good mechanical properties of steels. Carried out work is focused on the development and study of the microstructure and mechanical properties evolution while passing by the various thermomechanical treatments of two nuances of Molybdenum HSLA steels. EXPERIMENTAL PROCEDURE Melting is made in an induction furnace and aimed an HSLA steel with Molybdenum. The chemical composition obtained is on table 1. The thermomechanical cycle carried out is represented on figure 1. Table 1: Chemical composition.

C Mn Si P S Ni Cr Cu Al Ti V Nb Mo Steel A 0.11

0.12 1.19 1.22

0.21 0.22

0.008 0.011

0.005 0.04 0.05

0.06 0.17 0.18

0.034 0.045

0.015 0.019

0.07 0.0005 0.38 0.43

Steel B 0.11 0.12

1.21 1.24

0.23 /

0.009 0.01

0.005 0.008

0.05 0.09

0.06 0.19 0.22

0.034 0.047

0.02 /

0.08 0.0005 0.60 0.61

Figure 1: Thermomechanical cycle.

Homogenization

Temperature of deformation

800 °C 900 °C 1000 °C

Water Air


2

The ingots are homogenizing at a temperature of 1200 °C, followed directly by forging at various temperatures (800, 900, 1000 °C) with a red uction ratio of 66 % and two modes of cooling (Water, Air). The samples underwent a mechanical polishing carried out on abrasive papers of various granulometry, then completions on felts by using a diamond dust of 6 µm, 1 µm with a lubricating oil and an alumina paste of 0.05 µm, respectively. The various microstructures are highlighted by a chemical attack, whose reagent is the nital 5 % made up of 5 cm3 of nitric acid HNO3 and 95 cm3 of ethanol CH3 CH2 OH. The Vickers hardness tests HV10 were made by a durometer of mark INSTRON WOLPERT. RESULTS AND DISCUSSION Melting To control the quality of the various worked out ingots, we choose a radiographic test which, moreover revealed no harmful secondary shrinkage pipes considering their elimination during forging. A microscopic analysis illustrates a Widmanstätten structure for the two steels figures 2. This structure is often obtained in HSLA steels as-cast state. When proeutectoid ferrite exhibit a morphology consisting of needlike or plate-shaped grains, this structure is often referred as Widmanstätten ferrite. R.L.BODNAR and S.S.HASSEN [1, 2] showed, that in general, the volume fraction of the Widmanstätten structure increases with increasing prior austenite grain size and increasing cooling rate through the transformation range, but the first has a more significant effect. Although Widmanstätten ferrite usually nucleates at ferrite grain boundary allotriomorphs, it can also form at intragranular sites, such as non-metallic inclusions. The grain-boundary-allotriomorph-nucleated structures are termed Widmanstätten side plates. In contrast, the intragranularly nucleated Widmanstätten ferrite has been referred to as acicular ferrite [3].

Steel A Steel B

Figure 2 . Widmanstätten structure for the two steels after casting Controlled Forging The homogenized samples at 1200 °C undergo a forgin g at various temperatures (800, 900, 1000 °C), followed by two modes of cooling (Water, Air). The reduction ratio is 66 %. According to the variation of the thermomechanical treatment parameters (temperature of forging, modes of cooling), steels presents various microstructures. Steel A:

• Forging at 800 °C: o Water cooling : fine ferrite + small quantity of bainite o Air cooling : ferrite + pearlite

• Forging at 900 °C: o Water cooling : martensitic structure o Air cooling : ferrite + pearlite

X500 X500


3

• Forging at 1000 °C: o Water cooling : martensite + small quantity of ferrite o Air cooling : ferrite + small quantity of pearlite

The various metallographic structures of the steel A are illustrated on the figures 3, 4 and 5 according to the forging temperature and the cooling mode. For a forging at the temperature of 800 °C, HSLA st eel presents a fine ferritic structure and a small quantity of bainite and ferrite-pearlite, for water and air cooling, respectively figure 3a and 3b.

a) Water Cooling b) Air Cooling

Figure 3 . Microstructure after controlled forging at 800 ° C. (Steel A) It is estimated that such structures appear after a transformation process governs by a size control of the austenitic grain by the precipitates and a work hardening of the austenitic grain in a field of non-recrystallization. Thus for an accelerated cooling (water), ferrite inherits its fine structure, hardened by a carbonitride precipitation of Titane, Niobium and Vanadium which was delayed in austenite by Molybdenum [4]. For a forging at 900 °C, under the additional effec t of a good hardenability allotted to Molybdenum, a low temperature of transformation due to Manganese and a rapid cooling (water) supporting the formation of low temperature products, steel exhibit fine martensitic structure consolidated by a precipitation figure 4a. That gives the maximum of hardness; more the particles dispersed in the matrix obstruct the movement of dislocations (interactions dislocations-precipitates). Therefore the stress apply to cause their movements are more significant, which translated by an increase in the hardness figure 6. The transformation induced precipitation kinetic is observed to be faster when deformation is performed at this temperature. This is the consequence of a higher transformation temperature due to high cumulative strain of austenite in the deformation schedule [5]. For an air cooling, the formation of a ferrite-pearlite structure is favoured figure 4b. For a deformation at 1000 °C, after water cooling, the structure obtained is the result of the coalescence of recrystallized austenitic grains figure 5a. For air cooling the structure is ferrite plus small quantity of pearlite figure 5b.


Figure 4 . Microstructure after controlled forging at 900 ° C. (Steel A)

X500 X500

X500 X500


4


Figure 5 . Microstructure after controlled forging at 1000 °C. (Steel A)

Water cooling

287

345302

0

70

140

210

280

350

420

800 900 1000

Temperature °C

Har

dnes

s H

V10

Air cooling

168 172 175

0

30

60

90

120

150

180

210

800 900 1000

Temperature °C

Har

dnes

s H

V10

Figure 6. Influence of forging temperature on hard ness HV10. (Steel A) KOZASU and al. [6] claimed that not all deformation bands have the same ferrite nucleation potential and some bands have a poor nucleation capability. They also found that even after a very large deformation it was impossible to obtain a uniform distribution of deformation bands. Upon cooling, ferrite forms first at the easiest nucleation sites such as prior austenite grain boundaries and/or where the deformation is highest. Second phase nucleates at sites where nucleation is less easy. However, the size of second phase becomes smaller with decrease in finish rolling temperature due to the larger number of nucleation sites. Thus, it is concluded that at 1000 °C, there were less nucleati on sites of ferrite what gives a greater proportion of martensite. Steel B:

• Forging at 800 °C: o Water cooling : ferrite + bainite o Air cooling : ferrite + pearlite

• Forging at 900 °C: o Water cooling : acicular ferrite + small quantity of pearlite o Air cooling : ferrite + pearlite

• Forging at 1000 °C: o Water cooling : ferrite + bainite o Air cooling : ferrite + pearlite + small quantity of acicular ferrite

The various metallographic structures of the steel B are illustrated on the figures 7, 8, and 9 according to the forging temperature and the cooling mode. For the deformations at 800 °C and 1000 °C followed by water cooling, the structures consist of ferrite plus bainite with more smoothness for the first mode figures 7a and 9a respectively. The latter is the direct result of the low deformation temperature, which generates a fine structure. Moreover, the bainite is the result of the speed cooling and steel hardenability. This structure is a duel phase, one obtained at 800 °C i n the field (α + γ), the other at 1000 °C in the austenitic field. With a good hardenability and the addition of Molybdenum, we can obtain a structure dual phase, even after a deformation in the austenitic field.

X500 X500


5

At 900 °C, after forging and water cooling, steel h ad a very fine acicular structure figure 8a. It is the direct consequence of the deformation at low temperature, which generates a no recrystallized lengthened structure. Ti (N, C), Nb (C, N) and Molybdenum played their parts of recrystallization inhibiter in addition of intra granular germination of acicular ferrite on V (C, N) precipitate. For air cooling of the steel B, for all the temperatures of forging, the product remains long enough at the temperatures where the diffusion of carbon is still possible, thus allowing the formation of a ferrite-pearlite structure figures 7b, 8b, and 9b. The difference in hardness can be allotted to the different proportion of the pearlite and the ferrite grain size. The forging temperature has an important role on the grain size through the recrystallization phenomenon. For 800 °C and 1000 °C, ferrite seems t o be mixed between acicular and polygonal, thus contributing to the consolidation of steel.


Figure 7 . Microstructure after controlled forging at 800 ° C. (Steel B)


Figure 8 . Microstructure after controlled forging at 900 ° C. (Steel B)


Figure 9 . Microstructure after controlled forging at 1000 °C. (Steel B)

X500 X500

X500 X500

X500 X500


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According to the curves of hardness illustrated on figure 10, the optimum of hardness for water cooling is at the temperature of 900 °C and a t 1000 °C for the air cooling.

Water cooling

245260

240

100

160

220

280

800 900 1000

Temperature °C

Har

dnes

s H

V10

Air cooling

194176

228

100

160

220

280

800 900 1000

Temperature °C

Har

dnes

s H

V10

Figure 10. Influence of forging temperature on har dness HV10. (Steel B) CONCLUSION

• Molybdenum is exceptional steel alloying elements that not only imparts many unique and useful characteristics to steel but is also easy to add to the molten metal, melt losses are minimal.

• A microscopic analysis illustrates a Widmanstätten structure as-cast state. • The peak of hardness is observed for a forging at 900 °C, water cooling (steels A and

B). A hardening allotted to the presence of martensitic structure for Steel A and dual phase martensite ferrite for steel B.

• Ferrite grain size depends on the forging temperature. • With the Molybdenum, steel acquired a structure dual phase even after a cooling at

high temperature (in the austenitic field). • An addition of Molybdenum modifies the ferrite growth speed; so that the ferrite does

not mark any more the austenite grains joints, but precipitates as an intra granular acicular structure component.

• The effects of Molybdenum are the result of its action on ferrite in solid-solution and the composition, the formation and the distribution of carbides and slow down the formation of the pearlite.

REFERENCES [1] R.L. Bodnar and S.S. Hansen "Effects of Widmanstätten Ferrite on the Mechanical

Properties of a 0.2 Pct C-0.7 Pct Mn Steel", Metallurgical and Materials Transactions A, vol 25A, April 1994 – 763.

[2] R.L. Bodnar and S.S. Hansen "Effects of Austenite Grain Size and Cooling rate on

Widmanstätten Ferrite Formation in Low-Alloy Steels", Metallurgical and Materials Transactions A, vol 2A, April 1994 – 665.

[3] TADAAKI TAIRA and all "Development of Super Tough Acicular Ferrite Steel for

Line-pipe – Optimization of Carbon and Niobium Content in Low-Carbon Steel", Proceeding of International Conference on Technology and Application of HSLA Steels, October 1983.

[4] A.P. Coldren and all "Microstructures and Properties of Controlled Rolled and

Accelerated Cooling Molybdenum-Containing Line Pipe Steels", Proceeding of International Conference on Technology and Application of HSLA Steels, October 1983.


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[5] J.C. Herman, B Donnay and V Leroy "Precipitation Kinetics of Micro alloying Additions During Hot-Rolling of HSLA Steels", ISIJ International, vol. 32 (1992). [6] Nack Joon Kim, Gareth Thomas "Evolution of Multiphase Structures and their Influence on Mechanical Properties of Low Carbon Steels", Proceeding of International Conference on Technology and Application of HSLA Steels, October 1983.

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