THE EFFECT OF DYNAMIC LOADING ON THE STRUCTURE AND PROPERTIES OF 18G2A AND 14HNMBCu STEELS

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1 THE EFFECT OF DYNAMIC LOADING ON THE STRUCTURE AND PROPERTIES OF 18G2A AND 14HNMBCu STEELS J. Gronostajski, W. Palczewski To cite this version: J. Gronostajski, W. Palczewski. THE EFFECT OF DYNAMIC LOADING ON THE STRUC- TURE AND PROPERTIES OF 18G2A AND 14HNMBCu STEELS. Journal de Physique Colloques, 1985, 46 (C5), pp.c5-505-c < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1985 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSIQUE Colloque C5, supplgment au n08, Tome 46, aoot 1985 page C THE EFFECT OF DYNAMIC LOADING ON THE STRUCTURE AND PROPERTIES OF 18G2A AND 14HNMBCu STEELS J. Gronostajski and W. Palczewski Technical University of Wroc,hw, ul. PodwaZe 54 m. 7, WrocZaw, Poland R6sum6 - En utilisant les mat6riaux explosifs et le laminage, on a 6tudi6 l'influence de la d6formation des aciers 18G2A et 14HNMBCu sur leurs propri6t6s mgcaniques, leur microstructure et leur r6sistance 2 l'usure. On a constat6 que le changement des propri6t6s d&pend de la structure initiale ainsi que du type de la dgformation. Abstract - The ef6ect of deformation of 18G2A and 14HNMBCu steels by using explosive materials and by cold rolling on the mechanical properties, wear resistance and structure were investigated. It has been found that the changes of properties are dependent on the initial structure of steels and the mode of deformation. I - INTRODUCTION Numerous experimental studies have shown that the dynamic loading of materials leads to considerable changes in their mechanical properties and structure [l-51. And this is one of the main reason for an increasing application of explosive hardening in the production on a commercial scale. Investigations of metals under conditions of dynamic loading, besides their purely practical aspects, lead towards a better understanding of the mechanism of explosive hardening. At present there is a great number of theoretical publications on dynamic loading, where the materials are loaded mainly with a two-demensional wave. In spite of their complex mathematical description, the publications do not consider all the basic parameters of the process and do not take into account the effect of different initial states of the materials and therefore they cannot be used for practical purposes. In addition to that, in the practical application of explosive hardening of metals, one-dimensional pressure impulse is generally used. The desire of acquiring more accurate knowledge of the possibilities of work-hardening of HSLA steel and steel with higher amount of manganese after initial different heat treatment under the action of the onedimensional pressure impulse obtained as a result of deformation of an explosive was the purpose of the present investigations. The practical purpose is to determine the effect of dynamic hardening on the wear resistance of the steels. I1 - EXPERIMENTAL PROCEDURE The test materials 18G2A and 14HNMBCu steels were used in the form of rolled plates of 16x180~430 mm. The chemical analysis is given in Table 1. Article published online by EDP Sciences and available at

3 C5-506 JOURNAL DE PHYSIQUE Table 1. Chemical analysis The specimens of 18G2A steel were normalized at 1193K and specimens of 14HNMBCu steel were heat treated by quenching from 1233K and annealing at 923K. This caused the equiaxial grain size of 23 and ferrite -pearlite structure in 18G2A steel and 18 pm grain diameter and sorbite structure in 14HNMBCu steel. Samples were loaded dynamically by using MPW-8 plastic explosives with the charge thickness of 9, 15, 22.5, 36, 45 and 50 mm. A diagram of the set-up used for the dynamic loading of plates is presented in Fig.1. PCV foil ( 3) protected the loaded(4)sample from the formation of cavities and thermal ahock. Fig.1. Diagram of test set-up for dynamic loading: 1 - explosive, 2 - detonator, 3 - PCV foil, 4 - sample, 5 - bottom plate, 6 - base plate, 7 - ground In order to interpret the changes obtained in mechanical properties wear resistance and structure more correctly, they were compared with the same properties and structure of the steels deformed by cold rolling. Cold rolling samples of 16x30~200 mm were deformed by 3, 5, 10, 16, 20, 30, 40, 50, 60, 70 and 80%. For the applied load system and the explosive with an explosion rate of 7300 m/s, the maximum pressure at the explosive-specimen boundry was about 13.5 GPa and the rise in temperature was about 130 deg [5]. An increase of explosive thickness leads only to the prolongation of the pressure impulse duration; it does not affect the maximum value of the impulse pressure. The effect of thickness of an explosive and deformation by cold rolling on the strain-hardening, on the mechanical properties, on the wear resistance and on the structure was determined by means of the following tests: uniaxial tensile test, microhardness and hardness tests, impact resistance test, wear resistance test and metallographic tests by means of optical, scanning and transmission-5leftron microscopy. Tension was per- formed with the strain rate of 10 S using specimens of 125 mm gauge length, 30 mm width and different thickness. Brine11 hardness was determined by using a ball of 10 mm in dia and load of N. Microhardness was measured using a Leitz microhardness testing machine at a load of N. Impact resistance was determined by using Charpy hammer and specimens of 5x10 mm cross section and of 2 mm deep notch. Wear resistance was carried out on a trybometer testing machine in which the specimens were in contact with a counterspecimen made from hardened 0.45% C steel. The hardness of the counterspecimen was 4021 HRC. For preparation of foils, a wire saw was used to take slices of 0.4

4 thickness parallel to the deformed surface of specimens. These slices were mechanically polished to 0.1 mm thickness. Slices were then electropolished at the temperature of 2 85 i 287K, and bombarded. A Tesla BS;540 transmission electron microscope operating at 110 kv was used for the examination of the foils. Other structural observation were performed by means of Reichert MF microscope and a Stereoscan 180 scanning microscope RESULT AND DISCUSSION The effect of thickness of the explosive on the mechanical properties of 18G2A steel is given in Fig.2, and of 14HNMBCu steel in Fig.3. - UTSe I.... I PLASTIC STRAIN ( %) Fig.2. Effect of thickness of the explosive and of plastic strain obtained by explosion and cold rolling on the mechanical properties of 18G2A steel It follows from Fig.2 that the greatest hardening of 18G2A steel was obtained at the highest charge thickness used /45 mm/, which corresponded to the largest plastic deformation achiexed by explosion /15%/. The greatest hardening of 14HNMBCu steel was obtained at the smallest charge thickness /9 mm/, which coresponded to the smallest plastic deformation /2.5%/ /Fig.3/. A further increase in the explosive thickness over 9 mm caused a slight decrease of the strenght properties and an increase of plasticity. A drop of the flow stress occuring after an increase of deformation over 2.5% for 14HNMBCu steel makes it imposible to use the plastic strain as a measure of hardening and thus the flow stress cannot be expressed by conventional equations. To describe the flow stress as a function of strain, the factor of work softening connected with thermally actiuited processes should be introduced. This may be obtained by an expression of the flow stress as a function of the dislocation density. By comparing the changes of the mechanical

5 C5-508 JOURNAL DE PHYSIQUE properties of 18G2A and 14HNMBCu steel caused by plastic deformation; one can notice that the important factor is the initial structure of the steels. The tempering effect of 14HNMBCu steel caused by plastic work probably is larger than the hardening effect caused by straining. The 18G2A steel is not heat treated and the deformation causes only the strain hardening effect. By comparing the mechanical properties of explosively hardened samples with cold rolled samples it can be stated that for the similar values of plastic strains, the degree of the explosive hardening of the tested materials is distinctly greater than in the case of hardening by cold rolling. Table 2 shows the degree of strain of cold rolled materials which makes is posible to achieve the strenght properties equal to the maximum v a l ~ obtained s by explosive hardening. Fig.3. The effect of thickness of the explosive and of plastic strain obtained by explosion and cold rolling on the mechanical properties of 14HNMBCu S tee1. l...,... l PLASTIC STRAIN (a Table 2. Values of plastic strain at which the greatest strenght properties of explosively hardened materials are equal to those of cold-rolled materials Mechanical properties Ultimate tensile strenyht Yield stress /O. 2%/ Hardness HB Reduction in area Impact resistance rolling Plastic strain % 18G2A explosive 14HNMBCu rolling explosive

6 Examinations of the dislocation density of explosively hardened and of cold-rolled samples have shown that both the methods of deformation cause a significant increase of dislocation density in comparison with undeformed material. In explosively hardened samples of 14HNMBCu steel, the dislocation density reaches the maximum value at 9 mm explosive charge thickness,but for 18G2A steel explosively cold-rolled; the dislocation density increases with the degree of plastic strain. The dislocation density obtained by means of explosive hardening is distinctly greater than the dislocation density of both steels deformed in the same degree by cold rolling. This makes it possible to mderstand why the greatest explosive hardening was obtained at a significantly smaller plastic strain than that necessary for comparable hardening by cold rolling /Tab.2/. This also points to the importance of volume strain in the processes of explosive work hardening. The wear /W/ of the explosively loaded steels as a function of plastic strain from 0 to 15% for 18G2A steel and from 0 to 10% for 14HNMBCu steel, pressure from 0 to 2.4 MPa and slide velocity from 0 to 2 m/s between the specimens and counterspecimens is presented in the form of a polynomial. By means of the method of least squares and after rejection of insignificant coefficients on the probability level of 0.05, the following functions were determined. For 18G2A steel For 14HNMBCu steel where: E - plastic deformation in %, p - pressure in MPa, v - slide velocity in mfs. The effect of the plastic strain on the wear resistance of 18G2A and 14HNMBCu steels is a slightly differext. The wear of 18G2A steel linearly decreases with deformation caused by explosive, but wear of 14HNMBCu steel achieves a minimum value at the deformation equal to 4%, and at larger deformation is a litlle higher. The wear of both steels decreases with the increase of slide velocity in the investigated range. Pressure has a similar effect on the wear of 14HIWBCu steel, but in the case of l8g2a steel the effect of pressure in more complicated. The effect of pressure on the wear is depended on the slide velocity, at the higher velocity the wear decreases with increase of pressure, but at the lower values of velocity the effect of pressure is quite different. The wear resistance of explosively hardened steels is much higher than the wear resistance of cold rolling steels deformed in the same degree. It has been found, that the hardness of the surface layer of samples with good wear resisance is about twice bigger than the hardness of the inside part of samples. This can be explain as a effect of thermo-plastic treatment, cause by temperature at boundry between specimen and counterspecimen. The higher of slide velocity and pressure the higher temperature of surface layer of specimen and the bigger the dispersion hardening effect. IV - CONCLUSION The following conclusions based on the investigations of the explosive hardening of 18G2A and 14HNMBCu steel may be drawn: - The largest work-hardening was achieved for 18G2A steel at the highest

7 C5-510 JOURNAL DE PHYSIQUE thickness of plastic explosive 45 mm and for 14HNMBCu steel at the lowest 9 mm thickness, - The changes of properties of steel caused by explosion resulted from the superimposition of two processes: strain-hardening and work-softening connected with the tempering of dispersion hardened 14HNMBCu steel, - The explosively deformed steels have a greater dislocation density than cold rolled steels at the same degree of deformation, - The wear resistance of explosively hardened steels is higher than the wear resistance of cold rolled steels. REFERENCES 1. Antroszczenko B.S. et al.: Fizika Mietallov i Mietalloviedienye, 24 /1967/ =ski A., Thesis Ph.D.: Technical University of Warszawa, Stiepanov V.C., Sipilin P.M., Navagin J.S.: TZoczenie wybuchowe, WNT Warszawa, Gronostajski J., Garstka J.: Sheet Met-Ind. 60 /1983/ Dusek F.. Sbornik VAAZ, Brno, 4 /1963( 7.