UDK 669.14.018
Professional article/Strokovni članek
ISSN 1580-2949 MTAEC9, 45(5)483(2011)
ALLOYS WITH MODIFIED CHARACTERISTICS ZLITINE Z MODIFICIRANIMI LASTNOSTMI
Mirsada Gruč1, Milenko Rimac1, Omer Beganovic1, Sulejman Muhamedagic2
1University of Zenica, Instutute of metallurgy "Kemal Kapetanovic" Travnička cesta 7, Zenica, BiH 2University of Zenica, Faculty of Metallurgy and Materials Science, Travnička cesta 1, Zenica, BiH
miz@miz.ba
Prejem rokopisa - received: 2011-02-01; sprejem za objavo - accepted for publication: 2011-05-20
Industry has a permanent need for new materials to improve the performance of constructions. However, the development of new materials is a complicated and costly process, and in many cases the existing materials with modified characteristics are used. By the use of certain technological processes of producing liquid metals, deformation processes, thermal and surface treatments, the mechanical and exploitation characteristics of existing materials can be improved. In this paper is an overview of several research activities at the Institute in Zenica with the goal to improve the characteristics of some materials in use in the air and car industries. With semi-industrial facilities and in laboratories of the institute some researches where conducted on stainless steels and maraging steels as well as on superalloys based on nickel and iron. Key words: new materials, new technologies, stainless steels, maraging steels, super alloys
Industrija potrebuje nove materiale, če želi izboljšati lastnosti konstrukcij. Razvoj novih materialov je zapleten in drag proces in so zato mnogokrat uporabljeni materiali s spremenjenimi lastnostmi. Z spremembami v tehnologiji obdelave taline, procesov predelave, toplotne in površinske obdelave je mogoče izboljšati mehanske in uporabne lastnosti materialov. V tem deluje kratek pregled raziskav na Inštitutu v Zenici, ki so bile izvršene s ciljem izboljšanja materialov, ki so v uporabi v industriji vozil in letal. Na polindustrijskih in laboratorijskih napravah na inštitutu so bile opravljene raziskave na nerjavnih in maraging jeklih in na superzlitinah niklja in železa.
Ključne beside: novi materiali, nove tehnologije, nerjavno jeklo, maraging jeklo, superzlitine
1 INTRODUCTION
Modern industry has a permanent need for new materials to improve the performance of their constructions. The introduction and application of new materials for the improvement of existing structures with a higher level of exploitation properties is related to new material development and the mastering of these materials. In some cases designers, in cooperation with research and development institutions, use simple and economically reasonable solutions. One possibility is a modification (improvement) of some of the properties of existing materials with the aim to increase the exploitation and mechanical properties at room and elevated temperatures or improve the corrosion, heat and wear resistance. The term modification used here includes specific changes in chemical composition and microstructure, achieved with changes of technology during the production of liquid metal, hot, warm and cold processing and heat treatment.
This paper reviews several studies conducted on experimental equipment and in laboratories of the Institute of Metallurgy "Kemal Kapetanovic", University of Zenica, aimed at improving the properties of some special steels and superalloys used in the automotive industry: austenitic stainless steel, maraging steel and some nickel- and iron-based superalloys. Application of the remelting process in the case of maraging steel, the optimization of the chemical composition of austenitic stainless steel, and the application of modified rolling technology of the superalloy Nimonic 80A, lead to an
improvement of some properties. The application of metallic coatings on iron-based superalloy A286 also leads to a significant improvement of the exploitation properties.
2 APPLICATION OF REMELTING TECHNOLOGY IN MARAGING STEEL MAKING
High-strength maraging steels are strengthened to a high level with the coherent precipitation of the inter-metallic phase Ni3(Mo,Ti,Al) in a carbon-free, nickelmartensite matrix by aging at temperatures of 480 °C. They are intended to provide high values of tensile strength and typically have a high content of nickel, cobalt and molybdenum, but a very low carbon content. In fact, carbon and nitrogen are impurities limited to very low levels.
The high strength of maraging steel is achieved by the strengthening of the soft and ductile low-carbon nickel martensite with aging and the forming of a high density of very fine coherent intermetallic precipitates Ni3 (Mo, Ti, Al). The basic martensitic matrix with a high density of disoriented dislocations interacting with fine, uniformly distributed precipitates, forms a structure particularly resistant to local slip and premature breaking by stretching. The primary goal in making liquid mara-ging metal is to achieve a high-purity microstructure with a low content of non-metallic inclusions and parti-
cularly low levels of harmful elements such as carbon, nitrogen, sulfur and phosphorus. Carbon, nitrogen and sulfur tend to form brittle carbides, carbonitrides, sulfides and carbosulfides that could strongly reduce the absorbed impact energy. For this reason, the technology of liquid metal production is a process of fundamental importance for further processing and the achieving of the optimal mechanical and exploitation properties.
The elaboration of several maraging steel types (18Ni 200, 18Ni 250, 18Ni 300 and 18Ni 350) was developed in semi-industrial facilities and laboratories of the Institute and special attention was given to the manufacturing technology of 18Ni250 maraging steel. Respecting the highly complex final processing by means of cold flow turning, it was necessary to achieve the maximum purity and an extremely low content of carbon, nitrogen and sulfur and high levels of strength and ductility. These properties were achieved with remelting, especially electron-beam remelting 1.
The elaboration of the steel consists of three technologically related stages:
•	Melting the charge in open induction furnaces (OIP);
•	Remelting in vacuum induction furnaces (VIP);
•	Remelting under inert-gas-protected electric conductive slag (ETP) and under electron beam remelting (ESP);
Here, only the final results for remelted melts during ETP and ESP are presented.
Electrolytic iron, pure metals and the standard technology of liquid metal manufacturing in the OIP were used to obtain the basic charge. A high steel purity and a low content of undesirable elements were obtained by remelting of the basic charge in induction vacuum furnaces. The remelting on ETP led to a further reduction of the sulfur content and electron beam remelting to a further purity increase and a gas content reduction. In addition, both methods (ETP and ESP) achieve solidification grains very suitable for deformation. The maraging steel manufacturing and processing scheme, with basic reactions during molten metal processing is shown in Figure 1, and the chemical composition and mechanical properties are shown in Table 1.
Figure 1: Technological scheme of production with variations in the processes of melting (remelting) and deformation Slika 1: Tehnolo{ka shema proizvodnje s spremembami v procesih taljenja (pretaljevanja) in predelave
Figure 2: Microstructure of maraging steel: a) optical microscopy (500-times), b) SEM microscopy (2000-times) Slika 2: Mikrostruktura maraging jekla: a) opti~ni posnetek (pove~ava 500-kratna), b) SEM posnetek (pove~ava 2000-kratna)
The mechanical properties, especially the impact energy and fracture toughness K1C, clearly show the effects of remelting that allow very good further processing with cold working. In Figure 2 the maraging steel microstructure is shown. The examination in the optical and scanning electron microscopes showed that the microstructure consisted of a lath nickel martensitic matrix.
Table 1: Chemical composition and mechanical properties of maraging steel X2NiCoMo 18 9 5 (18Ni 250) according to WL 1.6354 (AMS 6514) in heat treated condition*
Tabela 1: Kemi~na sestava in mehanske lastnosti maraging jekla X2NiCoMo 18 9 5 (18Ni 2 50) po WL 1. 63 54 (AMS 6514) v toplotno obdelanem stanju*
Making
Ni
Co Mo
Content of elements, w/%c
Ti
Al
C
S
N
Rp0.2/ MPa
Mechanical properties
Rm/ MPa
A5/%
KV/J
K1J (MPam05)
Prescribed WL1 6354
17-19
8.09 5
4.65 2
0.60.9
0.050 15
max 0.03
max 0.010
max 0.010
max 0.03
0.0010 15
1910
1960
4.5
12
VIP+ETP
18.4
8.3
5.2
0.55
0.16
0.01
0.001
0.009
0.002
0.004
1905
1940
8.5
21.5
76
VIP+ESP
18.4
8.4
5.0
0.72
0.12
0.01
0.006
0.008
0.002
0.003
1893
1970
100
29.4
80.5
*Heat treatment: Triple quench from temperature 920 °C/water and aged at 480 °C/ 34 h/air
p
p
3 INFLUENCE OF NITROGEN ALLOYING ON THE MECHANICAL AND EXPLOITATION PROPERTIES OF AUSTENITIC STAINLESS-STEEL AISI 316
Austenitic stainless steel type AISI 316 has a very wide range of applications due to its good technological and exploitation properties. The possibilities of a variation of the chemical composition and the regime of technical processing for maintaining the austenitic structure and constant demands for cleanliness and corrosion resistance, have initiated an active research activity on this type of steel.
A significant role in these studies was played by the possibility of alloying with nitrogen for the replacement of part of the nickel content for the stabilization of the austenitic structure and the affect of nitrogen on the increase of the strength properties. A reduction of the nickel content has its economic justification and the increase in the yield and tensile strength is necessary for the production of structural components operating in aggressive media under high loads.
For the purposes of this research six experimental melts were produced in a vacuum induction furnace with three different variations of nitrogen content, i.e., (0.03, 0.06 and 0.12) %. For each variation of alloying with nitrogen, two melts were made with nickel contents of approx. 10.5 % and 13.5 %. This experiment enabled the simultaneous testing of the effect of alloying with
Figure 3: Yield and tensile strength of experimental melts depending on the nitrogen content and the heat-treatment condition (Q1 - 950 °C, Q2- 1100 °C)
Slika 3: Meja plastičnosti in raztržna trdnost eksperimentalnih talin v odvisnosti od vsebnosti dušika in pogojev toplotne obdelave (QI - 950 °C. Q2- 1100 °C
Figure 4: Elongation and absorbed energy of experimental melts depending on the nitrogen content and heat-treatment condition (Q1 -950 °C, Q2- 1100 °C)
Slika 4: Raztezek in absorbirana energija eksperimentalnih talin v odvisnosti od vsebnosti dušika in toplotne obdelave (QI - 950 °C, Q2 - 1100 °C)
nitrogen and to evaluate the possibility of reducing the nickel content within the limits prescribed for the steel AISI 316. The content of the basic elements and the achieved mechanical properties in the wrought and quenched conditions are given in Table 2 and in Figure
3	and 4. The obtained results show that alloying with nitrogen in amounts up to 0.12 % has a favorable effect on the strength properties of the steel AISI 316 2.
4	METALLIC ALLOYS FOR WORK AT ELEVATED TEMPERATURES
4.1 Austenitic stainless steel Nitronic 60
In recent years, based on conventional austenitic stainless steel 18/8, steels under the commercial name Nitronic, with the addition of manganese, nitrogen and silicon, have been developed. These steels are intended to operate at elevated temperatures and extended the exploitation area of austenitic stainless steel. In addition, it is significant that the austenitic structure is achieved with less-expensive stabilizing alloying elements, manganese and nitrogen.
Research results show that the occurrence of ö-ferrite in steel Nitronic 60 can be prevented with elements that stabilize austenite, nickel, and especially nitrogen, closer to the upper permitted limits, while the content of ferrite stabilizing elements is maintained in the middle of the allowed range. If the content of a ferrite stabilizing
Table 2: Content of the basic elements and mechanical properties in the forged and quenched condition of the experimental melts Tabela 2: Vsebnost baznih elementov in mehanske lastnosti eksperimentalnih talin po kovanju in gašenju
Content of elements*, w/%
Mechanical properties
Melts
Cr
Nl
N
ReH/ MPa
Forged (F)
rm/ MPa

KV/
J
Quenched 950 °C (F+Q1)
ReH/
MPa
Rm/ MPa

kV/
J
Quenched 1100 °C (F+Q2;
ReH/
MPa
Rm/ MPa

KV/
J
0 07
16 5
10.5
0 032
395
670
55
140
410
655
49
125
270
580
63
165
0.07
15.8
13.2
0 029
460
630
43
160
325
610
48
130
250
555
60
180
0.08
16.4
10.1
0.036
475
695
45
155
405
670
45
115
315
635
59
170
0.07
15.9
13.1
0.057
500
680
35
170
330
635
49
140
285
580
58
195
5
0.08
17.5
10.6
0.120
590
750
35
135
350
655
47
130
340
650
55
170
6
0.07
16.7
13.7
0.120
600
765
37
155
420
720
38
150
325
635
48
190
* The contents of other elements Mo, Mn, Si, S i P is approximately equal in all melts Materiali in tehnoloaije / Materials and technoloay 45 (2011) 5, 483-487
1
2

A
Table 3: Chemical composition of the experimental melts of steel S 218 00 (Nitronic 60) Tabela 3: Kemi~na sestava eksperimentalnih jekel S 218000 (Nitronic 60)
Making	Content of elements, w/%								Crekv1	Niekv2	^-ferrite3
	C	Mn	Si	P	S	Cr	Ni	N			
Prescribed ASTM A 276	max. 0.10	7.0-9.0	3.5-4.5	max 0.06	max. 0.03	16.018.0	8.09,0	0.080.18	-	-	-
Melt 1	0.06	7.0	3.6	0.002	0.015	16.5	8.1	0.113	21.9	16.8	3
Melt 2	0.07	7.8	4.2	0.002	0.012	18.0	9.0	0.109	24.3	18.3	8
Melt 3	0,08	7.8	3.6	0.001	0.012	16.8	9.0	0.178	22.2	20.6	0
NOTE:
1.	w(Crekv) = w(Cr) + 1,5 w(Si)
2.	w(Niekv) = w(Ni) + 30 w(C) + 30 w(N) + 0,5 w(Mn)
3.	According to revised Schaeffler constitution diagram3
Table 4: Chemical composition of experimental melts of superalloy Nimonic 80A Tabela 4: Kemi~na sestava eksperimentalnih talin superzlitine Nimonic 80 A
Variant	Content of elements, w/%										
	C	Cr	Si	Mn	Fe	Co	S	P	Ti	Al	Ni
Variant I	0.05	19.7	0.25	0.03	2.10	1.25	0,007	0.005	2.52	1.32	Balance
Variant II	0.04	20.8	0.23	0.16	1.48	1.30	0,006	0.005	2.68	1.44	Balance
element would be close to the upper limit to achieve the specific use characteristics, the content of the other ferrite stabilizing elements should be lower. Otherwise, the purely austenitic structure cannot be achieved, regardless of the content of the austenite stabilizing elements. The revised Schaeffler diagram 3 can be used to optimize the chemical composition at which the formation of ^-ferrite will be prevented 4.
4.2 Nickel-based superalloy Nimonic 80A
The superalloy Nimonic 80A is a nickel-base alloy intended for use at elevated and high temperatures where significant creep may occur. The primary strengthening mechanism of this superalloy is based on the precipitation of fine and coherent particles of intermetallic y' phase Ni3(Al,Ti) that increase the creep resistance. This strengthening mechanism for such a superalloy is more favorable than other strengthening mechanisms 5. The effect of hardening that can be achieved by the y' phase depends on the amount, dispersion, and size of the y' phase and it is controlled by heat treatment. The standard heat treatment includes a solution annealing at 1080 °C 8 h and precipitation aging at 700 °C 16 h. The maximal hardness of the superalloy Nimonic 80A achieved after this treatment is around 360 HV, but certain applications in the automotive industry require higher hardness. Since by long-lasting solution annealing at high temperature coarsening of grains occur, it is not possible to increase
the hardness (additional strengthening) significantly with a reduction of the grain size. The increase of the dislocation density after solution annealing and before precipitation aging, with cold or warm deformation, increases the strength, but the ductile properties are reduced significantly. A partial recrystallization after such a deformation results in a significant increase in the ductile properties and retains a high level of strength properties.
For additional strengthening of the superalloy Nimonic 80A the application of warm deformation is considerably more favorable than cold deformation, because the warm deformation can be done on a hot-rolling mill. In addition, the dislocation substructure obtained after the warm deformation is more favorable than that obtained after cold deformation, because the
Figure 5: Hardness (HV10) after corresponding treatment Slika 5: Trdota (HV10) po ozna~eni toplotni obdelavi
Table 5: Chemical composition and structure of the base material A286 and NiCrAlY metal powders Tabela 5: Kemi~na sestava in struktura osnovnega materiala A286 in prhov NiCrAlY
Material	Content of elements, w/%							Structure
	C	Ni	Cr	Mo	Al	Ti	Y	
A 286	0.05	26.0	15.5	1.3	0.2	2.1		y' + y + K
NiCrAlY	-	67	22.0	-	10.0	-	1.0	y'+y+Al2O3+Y3O3+iö
Figure 6: Applying metallic coatings with the HVOF process and the microstructure of superalloy A286 and coatings after the thermal treatment (solution annealing at 1080 °C and hardening at 740 °C for 8h)
Slika 6: Nana{anje metalnih prekritij z HVOF-procesom in mikro-struktura superzlitine A286 in prekritja po toplotni obdelavi (topilno zarjenje pri 1080 °C in utrditev pri 740 °C 8 h)
structure after cold deformation is characterized by chaotically distributed dislocations.
A change of the hardness of the two experimental melts (Table 4), subjected to solution annealing (1080 °C 8 h) after hot rolling, and then to 20 % warm deformation and recrystallisation annealing at different temperatures (960 °C, 1000 °C and 1040 °C in duration of one or two hours) and finally to precipitation aging (700 °C 16 h) is shown in Figure 5. The hardness of the alloy can be significantly increased through warm deformation after solution annealing. With the subsequent partial recrystallization the values of the hardness and the ductile properties can be controlled 5.
5 IMPROVEMENT OF THE CHARACTERISTICS OF IRON-BASED SUPERALLOYS WITH METALLIC COATINGS
A surface-enhanced structural element can be obtained with the application of surface-engineering technologies 6. The coated element combines the good properties of the base material and of the applied metallic layer. The combined properties could not be achieved using only one type of coating material. The selection of the base material and the metal coating is based on the requirements from manufacturers in the automotive industry that needed a cold-deformed iron-based superalloy A286 for some structural parts. However, the idea of rationalization with use of this as cast did not give satisfactory results, primarily because the required strength properties (hardness at the surface) could not achieved. The required surface hardness with
improved resistance to high-temperature corrosion was achieved by applying HVOF technology (High velocity oxyfuel) for forming metallic coatings (Figure 6). The process of applying the metallic powder NiCrAlY on the superalloy A286 was carried out by using a Diamond Jet technology in cooperation with the Development Department of "ORAO" Bijeljina.
The microstructural constituents of the base material A286 and the NiCrAlY coatings are given in Table 5. Aluminum and yttrium oxides improve the resistance to high-temperature corrosion and yttrium oxide particles act as a strengthening phase. The percentage of 40% of y' phase in the coating increases the surface hardness to values that exceed the hardness that can be achieved with the superalloy A286 7.
6	CONCLUSION
The producer's requirements ensured that particular structural elements satisfy the exploitation conditions were the starting base for this work. The concept of research included a preliminary analysis of the content and the interaction of the alloying elements, the structure analysis and the analysis of the relationship between the microstructure and the properties. Depending on the properties to be modified, the corresponding chemical composition and microstructure of the alloy, and then corresponding manufacturing technology, were designed. Well-known methods of designing (modeling) technologies were tested experimentally. In this article an overview of the modification of the properties of some materials suited for structural parts with improved performance is presented.
7	REFERENCES
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3	Metals Handbook: Properties and Selection: Irons, Steels and HighPerformance Alloys, 10th ed., Vol.1, ASM American Society for Metals, 1990
4Beganovic O., Muminovic B.: Optimizacija hemijskog sastava austenitnog nehrdajuceg celika Nitronic 60 u cilju sprecavanja nastanka ö-ferita, VII Naucno/strucni simpozij sa medunarodnim uce{cem METALNI I NEMETALNI MATERIJALI Zenica, BiH, 22-23. maj 2008
5Beganovic O., Oruc M., Rimac M., Uzunovic F.: Additional strengthening of superalloy Nimonic 80A, 14"" International Research/ Expert Conference Trends in the Development of Machinery and Associated Technology TMT 2010, Mediterranean Cruise, 11-18 September 2010
6	Hocking M. G., Vasantasree V., Siolky P. S.: Metallic and Ceramic Coatings, High Temperature Properties and Applications, John Willy and sons, INC., New York, 2000
7	Oruc M., Rimac M., Beganovic O., Delic A.: New materials as the base for development of modern industrial technologies, 13th International Research/Expert Conference Trends in the Development of Machinery and Associated Technology TMT 2009, Tunisia, 2009