A Y Barhoum - Improvement of some mechanical properties steel u8 via ammonia gas heat treatment - страница 1
ВІСНИК ДОНЕЦЬКОГО НАЦІОНАЛЬНОГО УНІВЕРСИТЕТУ, Сер. А: Природничі науки, 2013, № 1
IMPROVEMENT OF SOME MECHANICAL PROPERTIES STEEL U8 VIA AMMONIA
GAS HEAT TREATMENT
Faculty of Science, Tishreen University, Lattakia, Syria
This paper aims to improve some mechanical properties in the surface layers of U8 via ammonia gas heat treatment at a temperature range (600 to 950)°C where the nitrogen is introduced into the surface of U8, while it is in the ferrite state. The hardness was determined using Vickers tester with values ranged between 196-430HV. In addition, the forms of nitride phases are investigated using optical microscope and scanning electron microscope type JMS-6490LV. Furthermore, the corrosion resistance was studied after immersing it in seawater for 30 months where the mass loss was determined each three months.
Keywords: diffusion, nitriding, steel, gas, mechanical properties.
Introduction. Development of chemical and thermal treatment (CTT) of metals in the XX century followed the path of accumulation of experimental data on different types of technologies of this process. The scientific part of the study of CTT was limited to the traditional statement properties and structure of the diffusion layer. The mechanism of the formation of the diffusion with layer considered a unilateral mistaken point of view, based on the sequential process of structure formation in the diffusion of elements in the surface of the metal, i.e. in accordance with diagrams of the equilibrium state of the alloys. Insufficient attention was paid to physical and chemical nature of saturating environment that have a significant effect on the mechanism of the diffusion layer. In the study of conditions for the formation of the diffusion layer only in a few cases the equilibrium thermodynamics was involved The scientific interpretation of emergent properties in most cases limited to the study of the layer hardness , residual stresses, corrosion resistance and other properties. The absence of the CTT theory very brake as the development of science itself, and its implementation in practice.
The CTT theory must be based on a common approach to the process in each case in terms of the technological conditions of the process. In this connection should be allot great attention to the detailed study of physico-chemical and kinetic factors that form the diffusion layer. Necessary detailed study of the free metal surface (Gibbs surface), its structural and energetic state before and after the saturation of the diffusing element, as it will determine not only the physico-chemical properties of the diffusion layer, and the bulk properties of the metal. Creation theory CTT metals and alloys require attracting modern precision equipment .
Major development trends of nitriding at this stage are: 1) improving the nitriding process, providing reception of nitride layers , not prone to brittle fracture, and 2) the development of high-temperature nitriding processes that allow: a) accelerate the process of saturation, and b) to integrated layers consisting from the surface layer with the characteristics of the nitriding layers and downstream diffusion sublayer with the characteristics of the hardened layers; 3) development nitriding processes of and the nitriding, oxinitriding and carbonitriding under reduced pressure with using nitric oxide N2O as a gas-catalyst and oxidant, and 4) the further development of the processes of plasma (ion) nitriding and more modern processes of plasma carbonitriding; 5) the development of combined processes of saturation, improve the tribological characteristics of components (hardness and wear resistance) in dry sliding friction, 6) improving the nitriding powder alloys, and 7) improving the nitriding corrosion-resistant steels and other.
NH3 gas heat nitriding of the surface of tools, discs and parts of machines made of ferrite metals and alloys aims at forming harder protection layer to improve surface hardness, corrosion and fatigue resistance, and tolerance. Now, several methods to form protection layers of mechanical part surfaces are available: PDV, CVD surface laser coating, thermal depletion, ion implantation, treatment methods by surface melting using electron beams and others.
Proper change of the properties (mechanical, electrical, etc.) of metal products' surfaces is partly due to gas heat treatment, where both chemical and microstructure plus properties of surface layers change. This is ascribed to the reaction with ambient medium whether it is solid, liquid or gas by heating. As a result of this change the transitions in the phase change, and consequently the microstructure of these layers also changes.
The importance of this research is represented by increasing the hardness of surface layers in U8 carbon steel leaving the internal part as it is in terms of flexibilty, giving this kind of steel special importance for resisting corrosion, abrasion and generated stresses to be able to absorb shock during service. The objectives of this study are:
3. Study the resistence of considered samples to abrasion by immersing them in seawater for two and half years using the senstive scale Sartorius TE64 (60 g x 0.1 mg). Gaseous nitriding. In the stable Fe-N system and at equilibrium there are two Fe phases: a and у in forms of solid and gaseous solutions. The nitriding compounds (nitrides) are produced from N release under high pressure and and its accumulation in the defect zones, capillary interstices (pores), grain boundaries, etc. (fig.1) Collection in the first stage is in atomic form and then in molecular form. However, as the case in Fe - N system, the basic sense isn't ascribed to inequilibrium but to the stable (Fe - N) system [2 - 4].
Nitriding - a diffusive saturation process of steel surface layers with nitrogen. It takes place by heating steel in solid, liquid and gaseous media. The most common gaseous nitriding is in NH3 medium. Gaseous
nitriding increases microhardness, fatique resistance and resistance of surface solid layer against oxidization by air, water, vapour, etc.
Nitriding process in the ( Fe- N) stable sytem is accompanied by formation of several successive surface layers that contain two or more kinds of nitrides, and is composed of nitride phases in form of layers starting by a surface layer representing a mixture of (Fe2-3N) for
Fe4 N + [N ] — 2Fe2 N (є- phase) and Fe4 N for 4 FeN + [ N] — Fe4 N (у'- phase), followed by a separate layer of Fe4N for у' phase. The following layer is produced from eutectoid dissociation of у phase
according to y—>a+y' process. The last layser is a solid
, A. - . j-, Fig. 1. Schematic illustration of gas nitriding
solution from nitrogen in a-Fe . 0 00
Percentage and structure of nitride phases change depending on steel structure and alloys, where most doping elements (Mo, Cr, Mn and others) excluding (Ti, Si, Al) reduce N solubility in (Fe, M^3 N (є), and consequently N content and nitrded layer thickness are reduced. Existence of Ti, Si and Al increases the thickness of (у') phase layer plus formation of (Fe, M)4 N in form of continuous interlaced layer or uncontinous layers on the boundaries of shifts and grains that can reach a high depth casusing fragility of diffusion zone. The more nitride-forming elements in the steel and its alloys, the more dominant the (є) phase. On the other hand, high concentration of N in nitrided layer produces the fragile phase (£), (N) that is formed by cooling phase zones (є). In case
of nitriding steel in NH3 and saturation with oxygen and carbon, Fe2-3 (N, C) are formed and Fe2-3 (N, C, O) are accompanied by nitriding in a medium rich in N by brittlness of surface layers and appearance of pores, resulting in low plasticity of nitrided layers and cracks in surface layers and finally their destruction [2, 5, 6].
Formation of porous surfaces is consistent with stable phases in shift zones and grain boundaries and created vacancies, where different cracks containing N amounts are initially formed in atomic form under high pressure and then in molecular form. Pores are formed as single separate dots or as a system linked with successive and deep channels that cause deep damage to the surface and this damage can monitored by micromagnification .
According to the data of this research , the formed pores appear in the transitional phase (є) at concentration exceeding (8.15%N) post-cooling. This part of the layer remains single-phase at N concentration reaching (6.1 -7.15%N) in phase layer (є) plus absence of porosity, while phase (у) increases after slow cooling. As nitriding time increases the thickness of phase zone (є + у') doesn't change but phase cracks (є ) increase. According to the research data , cracks increase by increasing the thickness of phase zone є . Accordingly, increased carbon content in the steel sharply increases the thickness of porous layer. The walls of pore boundaries are mostly oxidized. Oxidation happens during cooling process of parts in the air , facilitating the oxidation and interaction of thin layers with sulphur.
Nitriding region is a polyphase one: solid solutions from the base-metal nitrides, nitrides and doping nitrides. Based on chemical activity of doping elements, two regions can be distinguished: I-type nitriding region and II-type nitriding region. Nitriding region is technically formed from the I-type in pure metals and alloys that contain low amounts of doping elements with low capacity for forming nitrides as compared to base metal. In this case, the intersurface diffusion region (nitriding region) is solid solutions from nitrides, including nitrides of intersurface diffusion region of base metal. As nitride density increases, concentration of nitriding regions able to grow near metal surface decreases. Moreover, the concentration of these regions increases in the volume and depth of diffusion region. I-type internal nitriding regions are characterized by low hardness number between
(150 - 300)HV , but they increase the tolerance of steel and its alloys. Il-type nitriding regions are formed in alloys where doping elements have more chemical affinity with nitrogen than with base metal. Here, nitrogen solubility increases noticeably and its diffusive motion is larger than that of doping elements. The inter-diffusive layer (secondary diffusion layer) represents II-type internal nitriding regions which are composed of solid solutions of Fe nitrides in the phase (y') and doping elements. Changes of solid solution are the reason beyond
formation of interrelated regions firstly then regions of "Ghener - Briston" type, followed by a separate region of independent intrinsic phases. Formation of nitrides results in low energy of free gypsium, particularly when the nucleons have round form (equilubrium state). As nucleon (crystal atoms) grows, the solubility or laminar regions are associated with nitride separation and loss of nitrogen for solid solutions and doping elements.
Research methodology. The samples were prepared from £/8 carbon steel (tab. 1) in form of rectangular parallelpiped (8x10x15) mm and put into quartz tube (11) for 30 min inside the heating chamber in normal electric tubular furnace at room temperature (fig. 2). Meanwhile, NH3 gas was pumped into the acting medium to release air from the system. NH3 nitriding was carried out on an initial set of samples in a quartz tube for 8 hours at temperatures (600°C, 800°C and 950°C). Temperature in the working zone was measured using electrothermal platinum duplicator with accuracy +10°C. The velocity of gas flow into the acting medium 20 cm3/min post-treatment left the samples inside heating chamber to cool to the room temperature.
Chemical composition of U8 carbon steel
Mn Ni S P
0.17-0.33 <0.25 <0.028 <0.03
After getting out the samples, the cross section was polished well to measure the microhardness and then to develop their surface with a mixture of ethanol (98%) and HNO3 (2%) for 20 sec.
Fig. 2. Diagram of the device applied in gaseous physio-chemical treatment (thermal) of samples (1) NH3 gas cylinder, (2) different gas cylinders (for compound gaseous treatment), (3,4) valves, (5, 6,7) control valves, (8,9) gas pressure gauges, (10) electric heating unit, (11) a quartz tube, (12) samples, (13) electric feeding unit and (14) water pond (reverse controller)
Results and discussion. Fig. 3 shows the results of microhardness distribution in function of diffusion layer thickness in U8 carbon steel after NH3 gas heat treatment at temperatures (600°C, 800°C and 950°C) for 8 hours. It has been revealed that the microhardness at 600°C is ranging between 196 HV and 430 HV. Microhardness approximately increases 2.5 times due to the phase (є), and this is related to the generated pores i.e. the microhardness between porous zones and holes filled with molecular nitrogen. As temperature rises to 800°C, the microhardness ranges between 150 HV and 430 HV.
Major contribution to increasing microhardness at a nearly rate of three times is approximately due to the phase (y'), [solid solution of nitrides (N)]. Low interference on the surface layer of hardness
untreated at RT
distance to the surface, mm
Fig. 3. Changes of hardness in function of hardened layer depth in U8 carbon steel after NH3 gas heat nitriding
distribution curves at different temperatures plus remarkable decline of hardness near the surface layer at 800°C are noticed. When nitriding temperature rises to 950°C the phase (є) practically gets absent, and the microhardness ranges between 150 HV and 350 HV. This is in agreement with the research results [5, 7, 8]. Therefore, and due to non-formation of a uniform layer from (Fe -a ), but a porous layer from the phase є , the nitriding process goes on as pores and holes, in this case, are filled with molecular nitrogen under pressure reaching 30 atm., i.e. it represents reservoirs for supplying with nitrogen moving between pores and holes, resulting in continual nitriding even after stopping NH3 pumping into the middle of treatment.
To certify this data, we studied the microstructure of U8 carbon steel using the metalizing microscopic МИМ-7 (fig. 4, a) that shows the change of microstructure at temperature 950°C after nitriding for 8 hours.
є , у', у' + a , a
Fig. 4. a - microstructure of U8 carbon steel after NH3 gas heat nitriding for 8 hours at temperature 950°C (x 300); b - shows the distribution of doping elements during diffusion layer thickness after nitriding process. nitrogen (1), carbon (2), oxygen (3)
Fig. 4, b illustrates the distribution of doping elements during diffusion layer thickness after nitriding process, while fig. 5 shows the resistance of U8 carbon steel against corrosion in seawater for 30 months after NH3 gas heat treatment. Conclusions.
1. The effect of NH3 gas heat treatment on properties and measurements of interactive diffusion layer in U8 carbon steel as a result of complicated processes occur in the phase є + у', pores and holes filled
with nitrogen and others was obtained. This increased the thickness of such layer from 1.5 to 3 times.
2. It has been obtained that the NH3 gas heat nitriding in function of thermal factor leads to an increase in the microhardness of diffusion layer of 2.5 times, keeping the sample interior with its elasticity to be able to absorb external shocks and stresses.
3. There is a discrepancy in the resistance of sample corrosion after nitriding by its immersing in seawater for two years. This is due to inconsistency of surface nitriding.
4. The probable existence of closed pores filled with molecular nitrogen in the phase є and the effect of oxygen on gas heat treatment process need further study.
Метою даної роботи є покращення механічних властивостей поверхневих шарів сталі У8 в результаті термічної обробки в аміаку при температурах (600-950)°С, яке має місце за рахунок проникнення азоту щодо поверхневого шару У8, який вміщує феритну фазу. Твердість, яку було виміряне за методом Вікерса, має діапазон у межах 196-430 HV. Утворення нітрид них фаз спостерігали за допомогою оптичного мікроскопу і сканую чого електронного мікроскопу JMS-6490LV. Опір корозії вивчали в морській воді протягом 30 місяців з шагом у 3 місяці
Ключові слова: дифузія, азотування, сталь, газ, механічні властивості.
untreated U 8 at RT 8h, NH U8,T=600 С 8h, NH U 8 ,T=800'c 8h, NH U 8,T=950 С
15 20 three month period
Fig. 5. Resistance of U8 carbon steel after corrosion in seawater for 30 months after NH3 gas heat treatment
Целью настоящей работы является улучшение механических свойств поверхностных слоев стали У8 в результате термической обработки в аммиаке при температурах (600-950)°С, которое происходит за счет проникновения азота в поверхность У8, содержащую ферритную фазу. Измеренная твердость по методу Виккерса оказалась в пределах 196-430 HV. Образование нитридных фаз наблюдалось с помощью оптического микроскопа и сканирующего электронного микроскопа JMS-6490LV. Сопротивление коррозии изучалось в морской воде в течение 30 месяцев с шагом 3 месяца.
Ключевые слова: диффузия, азотирование, сталь, газ, механические свойства.
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Received March 11, 2013
Determine the microhardness of considered samples of U8 carbon steel after NH3 gas heat treatment using Vickers method and microhardness ПМТ-3 tester.
2. Study the microstructure after NH3 gas heat treatment using Optical microscope MHM-7 and the electronic scanner Scanning microscope JSM-6490L V.