INVESTIGATION OF THE EFFECT OF NICKEL CONTENT ON THE ... · Nosov Magnitogorsk State Technical University Lenin Street, 38, Magnitogorsk city, Chelyabinsk Region Russian Federation,

Pavel P. Poletskov, Marina S. Gushchina, Olga A. Nikitenko, Alla S. Kuznetsova,Natalia V. Koptseva, Yuliya Yu. Efimova, Dmitry M. Chukin

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INVESTIGATION OF THE EFFECT OF NICKEL CONTENT ON THE STRUCTURAL AND PHASE TRANSFORMATION AND PROPERTIES OF HIGH-STRENGTH

COLD-RESISTANT COMPLEX-ALLOYED STEEL


ABSTRACT

It is established the effect of nickel content in medium carbon complex-alloyed steel on structural and phase transformation, critical points position, structure quantitative parameters and critical quenching rate. Continuous-cooling transformation diagrams of overcooled austenite decomposition in steel with original composition are presented. Typical steel microstructures which are formed at cooling with different rates are described. The positive effect of nickel content on a complex of rolled plate mechanical properties after quenching and low tempering is shown. The results of the investigation can be used for the design of high-strength cold-resistant steels compositions and regimes of their thermal treatment.

Keywords: high-strength cold-resistant steel, complex alloying, nickel, microstructure, phase composition, me-chanical properties.

Received 13 December 2018Accepted 12 July 2019

Journal of Chemical Technology and Metallurgy, 54, 6, 2019, 1291-1297

Nosov Magnitogorsk State Technical University Lenin Street, 38, Magnitogorsk city, Chelyabinsk RegionRussian Federation, 455000E-mail: [email protected].

INTRODUCTION The global development trend in the design of mate-

rials composition for different applications is creature of special steel compositions with high strength and hard-ness. The use of thinner but more strength steel sheets makes it possible to decrease the total weight of cars and constructions, increase the handling equipment carrying capacity with saving the weight of the whole vehicle, decrease the welding materials consumption [1 - 4].

Development of natural resources in near-polar regions and Arctic shelf of the Russian Federation predetermines the necessity of application of high-strength steels with hardness not less than 500 HBW for manufacturing of vehicles and heavy-loaded welded constructions which are exploited at temperatures up to -40°С [5 - 8]. Materials for manufacturing of modern mechanisms, exploited at low temperatures, must have the incompatible complex of mechanical properties: to be sufficiently strength maintaining high loadings and

simultaneously have rather high ductility and toughness for preventing the risk of brittle damage [9 - 12].

The difficulty for ensuring such unique properties combination is due to the fact that as a rule with higher strength level the toughness and crack resistance de-crease. Exploitation of vehicles and constructions at low temperatures also is accompanied with ductility and toughness decrease [13 - 14]. Ensuring of high strength without significant degradation of viscosity and ductility is possible by design of appropriate steel chemical compositions as well as course of controlled phase transformations, aimed at forming of the desired microstructure in rolled steel plates during thermal treatment [15].

It is well known that using nickel for alloying low carbon steel results in increase of toughness and decrease the critical temperature of cold brittleness [14]. Besides, nickel is considered to be the element which allows to improve the steel hardness to high extent [16 - 21]. Effect of nickel is explained by weakening interstitial

Journal of Chemical Technology and Metallurgy, 54, 6, 2019

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interatomic bonds in particular with carbon and dislo-cations. It facilitates the course of plastic deformation elementary acts. Under influence of nickel the initial microstrains distribute uniformly in the grains’ volume taking into consideration the presence of lattice defects, grains boundaries and other inhomogeneities [14].

The aim of the study is to investigate the nickel effect on the structural and phase transformations and proper-ties of high-strength medium carbon complex-alloyed steel with hardness not less than 500 HBW.

EXPERIMENTAL

The feature of this investigation consists in using the scientific and research complex equipment of OJSC “Engineering center Thermodeform-NMSTU” (Magni-togorsk, Russia) which simulates the real processes of steel manufacturing and rolling.

Ingots of medium carbon complex-alloyed steel with 0.9 %, 1.6 % and 3.2 % nickel content were produced in vacuum induction furnace (Table 1).

Reduction of ingots with 300 mm in height was car-ried out on hydraulic press (rough stage) and single-mill reverse rolling mill 500 “DUO” (finish stage). Prelimi-nary ingots were heated up to 1200°С, temperature at the end of the rolling was 850 - 950°С. Rolled metal final thickness reached 8 mm. Rolled plate was air cooled.

Quenching in water from 980°С with further temper-ing at 200°С and cooling in still air were used. Metal-lographic investigations were performed in the Centre of Joint Ownership of Scientific and Research Institute “Nanosteel” of Nosov Magnitogorsk State Technical University (Magnitogorsk, Russia).

For investigation the overcooled austenite decom-position in steels under study (Table 1) the samples with 10 mm in diameter and 80 mm in length were austenized using Gleeble 3500, vacuum heated up to 980°С at the rate 1°С/sec with further exposure for 15 min and cooled

at different rates in the range from 1 to 20°С/sec. Based on the obtained results the dilatometric curves were plotted which showed the relation between the sample diameter and the temperature. Inflections of curves cor-responded to critical points.

Metallographic sections for microanalysis were made from samples in accordance with standard method. Surface was etched by sinking into a bath with 4 % solution of nitric acid in ethanol. Qualitative and quan-titative characteristics of microstructure were studied by optical microscopy using Zeiss Axio Observer 3 microscope and Thixomet PRO software for image analysis. Microstructure was observed by scanning electron microscopy using JSM 6490 LV microscope at 1000 times magnification.

Mechanical tests of the rolled samples after heat treatment (quenching and low tempering) were per-formed in accordance with standards:

- tensile testing, ASTM E8 / E8M-16a;- impact testing, ASTM E23, at temperature -40°С

on the transverse V-notched test-piece 10 mm x 5.0 mm;- Brinell hardness test, ASTM E10-17, under loading

29430 N on the depth of 2 mm under the sample surface using the carbide ball with 10.0 mm in diameter.

RESULTS AND DISCUSSION

Analysis of samples microstructure showed that after cooling at rate 1°С/sec in steel with 0.90 % and 1.60 % Ni content mainly the bainite is observed (ap-proximately 80 - 93 %), low quantity of martensite remains (Fig. 1 a, b).

For steel with 3.20 % Ni content already at low cooling rates (1°С/sec) mainly martensite (till 93 %) and low quantity of bainite (till 5 %) are observed in microstructure (Fig. 1 c).

After cooling at rate 3°С/sec bainite quantity sharply decreases (for steel of melts 1 and 2 to 7 - 10 %, for

Table 1. Chemical composition of steels under study.

Number of melt

Content of chemical element (wt %) Critical

temperatures (°C)

C Si Mn P S Cr Ni Mo В Ac3 Ac1 1 0,28 0,2 0,8 0,007 0,001 0,5 0,9 0,3 0,003 800 735 2 0,29 0,2 0,8 0,010 0,002 0,5 1,6 0,3 0,003 790 720 3 0,28 0,2 0,8 0,007 0,001 0,5 3,2 0,3 0,003 750 695


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Fig. 1. Microstructure of medium carbon complex-alloyed steel after cooling at rate 1°С/sec: a - steel of melt number 1 with 0.90 % Ni; b - steel of melt number 2 with 1.60 % Ni; c - steel of melt number 3 with 3.20 % Ni (B - bainite; M - martensite).

Fig. 2. Microstructure of medium carbon complex-alloyed steel after cooling at rate 5°С/sec: a - steel of melt 1 with 0.90 % Ni; b - steel of melt 2 with 1.60 % Ni; c - steel of melt 3 with 3.20 % Ni (B - bainite; M - mar-tensite).

a)

b)

c)

a)

b)

c)


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Number of melt

Cooling rate, °C/s

Relative volume fraction of structural components,%

Hardness, HV

Hardness, HBW

Bainite (B) Martensite (М) + retained austenite

1 1 93 7 324 304

3 10 90 476 456

5 7 93 491 466

10 5 95 497 475

20 0 100 503 485

2 1 80 20 358 342

3 7 93 480 456

5 5 95 495 475

10 0 100 532 504

20 0 100 533 504

3 1 7 93 474 447

3 5 95 536 513

5 0 100 545 523

10 0 100 549 523

20 0 100 552 532

Table 2. Quantitative characteristics of the microstructure and hardness of steel of the studied melts at various cooling rates.

steel of melt 3 to 5 %) (see Table 2), and microstructure consists predominantly from martensite.

In steels with 0.90 % and 1.60 % nickel content at higher cooling rates (5°С/sec) along with martensite the low quantity of bainite remains (5 - 7 %, Fig. 2 a, b). For steel with 3.20 % Ni content (steel of melt 3, see Table 1) at cooling rate 5°С/sec bainite is not observed and microstructure fully consists of martensite and retained austenite (Fig. 2 c).

After cooling at rate 10°С/sec bainite structural component is observed only in steel of melt number 1 (Fig. 3). After cooling at rate 20°С/sec and higher the microstructure of all steels under investigation fully consists of martensite and retained austenite.

In Fig. 4 results of quantity of martensite (a) and

hardness (b) which are formed in investigated complex-alloyed steels at different cooling rates are presented.

Based on the complex investigations results the continuous-cooling transformation (CCT) diagrams of overcooled austenite in steel of melts 1 - 3 were plotted (Fig. 5).

Comparative analysis of CCT diagrams showed that additional alloying with Ni from 0.90 % to 3.20 % results in increase of overcooled austenite stability, decrease Ас3 temperature by 50°С, Ас1 temperature by 40°С, and improves hardness at all cooling rates. Desired hardness level (more than 500 HBW) was reached after cooling rate not less than 20°С/sec for steel with 0.90 % Ni, 10°С/sec for steel with 1.60 % Ni and 3°С/sec for steel with 3.20 % Ni (Fig. 4, b). The increase of hardness


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Fig. 3. Microstructure of medium carbon complex-alloyed steel after cooling at rate 10°С/sec: a - steel of melt 1 with 0.90 % Ni; b - steel of melt 2 with 1.60 % Ni; c - steel of melt 3 with 3.20 % Ni (B - bainite; M - mar-tensite).

a)

b)

c)

Fig. 4. Effect of cooling rate on relative quantity of Mar-tensite (a) and Hardness (b) of steel with different Ni content.

Fig. 5. Continuous-cooling transformation diagrams of overcooled austenite decomposition in medium carbon complex-alloyed steel with different nickel content: 1 - steel of melt 1 with 0.90 % Ni; 2 - steel of melt 2 with 1.60 % Ni; c - steel of melt 3 with 3.20 % Ni (A- austenite; B - bainite; M - martensite).

a)

b)


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value can be explained by existence of big amount of martensite even at low cooling rates.

Results of mechanical tests of steel samples after quenching and low tempering prove that increase of Ni content from 0.90 % to 3.20 % in medium carbon complex-alloyed steel makes it possible to improve plate strength, tough and ductile properties. Impact strength at -40°С raised on 42 % when relative elongation grew on 18.5 % (Fig. 6.).

CONCLUSIONS

Results of complex investigations of structural and phase transformation of overcooled austenite and me-chanical tests makes it possible to conclude that increase of Ni content from 0.90 % to 3.20 % in medium carbon complex-alloyed steel ensures the following aspects:

• significant increase of overcooled austenite stabil-ity and decrease of Ас3 temperature by 50°С;

• attaining necessary values of hardness (more than 500 HBW) at lower cooling rates (3ºС/sec) which can be explained by martensite formation even at these cooling rates;

• decrease of critical quenching rate from 20°С/sec to 5°С/sec. This fact is of high practical significance for high-strength plate manufacturing in industrial condi-tions because allows to exclude in the perspective the purchase of expensive equipment (high capacity quench-ing machines which enable the high values of cooling

rate at quenching);• simultaneously increase strength, tough and ductile

rolled plate properties (raise of impact strength at -40°С on 42 %, growth of relative elongation on 18.5 %).

AcknowledgementsThe study was financially supported by Ministry

of Education and Science of the Russian Federation within the scope of accomplishment of multiple-purpose projects of creating modern high-tech production with the participation of higher education institutions (Con-tract 03.G25.31.0235).

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Fig. 6. Mechanical properties of laboratory samples of high-strength cold-resistant steel with different Ni content after heat treatment.


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INVESTIGATION OF THE EFFECT OF NICKEL CONTENT ON THE ... · Nosov Magnitogorsk State Technical University Lenin Street, 38, Magnitogorsk city, Chelyabinsk Region Russian Federation,