RESEARCH ON DEGRADATION CORROSIVE ENVIRONMENT OF SOME STEELS USED IN MANUFACTURING MINING EQUIPMENT. MECHANICAL TESTS

Size: px
Start display at page:

Download "RESEARCH ON DEGRADATION CORROSIVE ENVIRONMENT OF SOME STEELS USED IN MANUFACTURING MINING EQUIPMENT. MECHANICAL TESTS"

Transcription

1 RESEARCH ON DEGRADATION CORROSIVE ENVIRONMENT OF SOME STEELS USED IN MANUFACTURING MINING EQUIPMENT. MECHANICAL TESTS Lecturer PhD. eng. Florin CIOFU, Engineering Faculty, Constantin Brâncuşi University, Lecturer PhD. eng. Alin NIOAŢĂ, Engineering Faculty, Constantin Brâncuşi University, Abstract: Simultaneous action of mechanical stress variables and the corrosive environment leads to the deterioration of structures made of steel or of other metallic materials, a process called fatigue in corrosive environment or corrosion fatigue, which has causes and effects different from degradation under the singular action, either of mechanical strain or of corrosive environment. The aim of this paper is the study of the behavior,under fatigue conditions in corrosive environment of low alloy steels meant for the construction of mining equipment used in various mining operations in Gorj county. Based on experimental research regarding sustainability in fatigue corrosion, analyses of the interaction between the mechanical and the electrochemical factors as well as microscopic analyses of the structures we aim in this paper at deeply studying the performance of degradation processes that occur in the surface layers and lead to destruction by corrosion fatigue of the steels under study. Key words: degradation, corrosion, strength 1.INTRODUCTION Degradation of a material is characterized by any change in its state relative to its original condition. Making the structural analysis by the designer requires knowledge of induced stresses in the components materials, of modes of degradation during manufacturing and use. A study made by the American Society of Civil Engineers has determined that 80-90% of the damage occurring in steel structures are caused by processes of fatigue, representing a process of side cumulative breakage caused by cyclic variable loads. If the fatigue phenomenon occurs in the presence of an aggressive environment, this type of degradation is known as the corrosion fatigue. The degradation through corrosion may reduce the strength to fatigue of the material as it shrinks during the initiation of fatigue cracks on the surface of the material [1]. All the materials based on Fe, Al, Ti, Cu or other ferrous or nonferrous metallic materials, are liable to this way of destruction. Fatigue in corrosive environments raises special economic and technical issues in the consequences they can cause as a result of accidental damage of the constructive elements in the structure of the machines and installations in various industrial fields. 89

2 2.EXPERIMENTAL METHODS USED 2.1. Materials under study The steels for the structures of surface mining equipment are used either in welded construction or assembled by rivets or screws and are chosen from the category for general use STAS 500 / 3-80 constructions, respectively AFNOR / A36101 A35501 EURONORM For the main components of metal construction excavators are used thick steel sheets whose technical delivery conditions are specified by ISO CR SR: Compared to steels for general use in the SR CR ISO 15608: 2002 standard are indicated two steel brands, OL 37 4k (E24-4) and OL 52 4k (E36-4) having improved chemical composition, mechanical and technological properties in the sense of increasing the weldability properties of the material. The improvements relate primarily to limiting the sulfur content, on product at 0.030% for OL 37 4k (E24-4) and at 0.020% for OL 52 4k (E36-4) as compared to 0.045% of STAS 500 / 3-80.The OL 4k 37 (E24-4) steel brand is deoxidyzed with silicium and aluminum and the steel 4k 52 OL (E36-4) one with silicium, aluminum and titanium. For reducing the phenomenon of lamellar teasing, the sheets of OL 4k 52 (E36-4), more than 20 mm thick, used for the required elements in its thickness direction must also comply with the conditions of Z 15 quality class, according to EN : Also for the sheets of OL 4k 52 (E36-4) brand, the value of the equivalent carbon computed on liquid steel formula is guaranteed with formula [2]: General purpose steels used for making metallic structures have a low percentage of carbon, not exceeding 0.25%. Steel mark is defined generally according to the minimum value of tensile strength, as evidenced by a system of notation indicating: -The domain of steel, specified by a literal symbol, ie general use OL-steel for construction, delivered as plastically deformed products in hot working in laminated semi - finished products and forged bars. -Minimum tensile strength in dan/mm2 for general purpose construction steels (OL), cast steel (OT) and steel for pipe (OLT). - Quality class, marked by numbers from 1 to 4, indicates the chemical composition and the mechanical strength and technological characteristics guaranteed on steel delivery Mechanical and chemical composition of steels under study E36 steel is used in construction of mining equipment, especially in manufacturing porters in the central area, tower and arms of heavy excavators. The main mechanical characteristics and the chemical composition of this steel are shown in Tables I and II. (1) 90

3 TABLE I. Mechanical characteristics of E36 steel (according to STAS ) Material Mechanical characteristics σ r (N/mm 2 ) σ c (N/mm 2 ) Elongation at break (%) E36 steel where: σ r =tensile strength, σ c = flow limit TABLE II. Chemical characteristics (share %) of E36 steel (according to STAS ) Chemical compostion (%) Chemical compostion (%) C 0,18 Cu 0,01 Si 0,28 Ni 0,01 Mn 1,5 Cr 0,01 P 0,018 Mo 0,002 S 0,008 Nb 0,04 Al 0,03 B 0,0001 Ti 0,004 Ca 0,0001 V 0,06 E24 steel is used in the steel load-bearing welded structures of medium-duty mining equipment. The main mechanical characteristics and chemical composition of this steel are shown in Tables III and IV. TABLE III. Mechanical characteristics of E24 steel (according to STAS ) Material Mechanical characteristics σ r (N/mm 2 ) σ c (N/mm 2 ) Elongation at break(%) E24 steel where: σ r = tensile strength, σ c = flow limit TABLE IV. Chemical characteristics (share %) of E24 steel (according to STAS ) Chemical compostion (%) Chemical compostion (%) C 0,19 Cu 0,015 Si 0,09 Ni 0,015 Mn 0,85 Cr 0,018 P 0,045 Mo 0,002 S 0,045 Nb 0,04 Al 0,022 B 0,0001 Ti 0,004 Ca 0,0001 V 0,04 The welded specimens have been achieved with a V butt welding with completion, with electrode basic type E 7018 (ISO 2560: E 51 5 B H) for high-carbon and low alloy steels following mechanical and chemical properties (Tables V and VI). 91

4 TABLE V. Mechanical characteristics of welding belt Welding Mecanical characteristics material Tensile Strength σ r (N/mm 2 ) Flow limit σ c (N/mm 2 ) Impact resistence (J) Elongation break (%) at TABEL VI. Chemical composition of welding belt Chemical C Si Mn composition Content (%) 0,07 0,5 1,0 Welding electrodes made of coated base is highly resistant to cracking both in hot and cold working Geometry of specimens Attempts at fatigue through plane bending were made on smooth specimens (Fig.1) welded specimens (Figure 1b) and with stress concentrator (Fig. 1c, 1d). The quality of welding belts in the case of welded specimens was verified by a nondestructive X-ray check in order to detect any discontinuities that may constitute chevrons. a smooth specimen b. welded specimen epruveta sudată, a V butt welding with completion c. Stress concentrator with rounded tip d. Stress concentrator in V Fig. 1. Geometry of specimens 92

5 2.4. Experimental stand for tests for fatigue through bending Experimental stand is a testing machine for fatigue through bending, flat with cantilevered specimen (Fig. 2) which allows doing tests with different degrees of asymmetry. In the paper tests were carried out in a symmetrical alternating cycle (R=-1) at a speed n=750rot/min. Fig. 2. Machine for fatigue through flat bending The test bench (Fig. 2) consists of: drive motor (P = 2.2kW, n = 750 rot/min), coupling, eccentric gear, leverage, device for gripping the specimen, console specimen, specimen constraint system. The fatigue tests in corrosive environment have been carried out on the same machine under test, spraying an aqueous solution of 3.5% NaCl and 6% H 2 SO 4 on the samples. 3. EXPERIMENTAL RESULTS THAT WERE USED From measurements made on smooth steel specimens brand E36, respectively E24 in corrosive environment, respectively in the air, curves of sustainability were traced in log coordinates σ - log N (Fig. 3 a, b). It was found that the smooth specimens show greater durability in the air than in corrosive environment. Simultaneous operation of a further mechanical stress with a chemical factor lead to a deterioration quicker than it would have been if the specimen had been subjected separately to a cyclical effort in the air or a simple corrosive action [3]. Under the influence of variable loads of protective oxide films, breaking leads to local anodic dissolutions of metal and to pitting corrosion. Corrosion pinching can be considered stress concentrators and are chevrons for fatigue cracks. As a result of stress concentration also occurs an intensification of local deformations that may be higher than those considered by the nominal value of tensions. Additionally, due to local strains of inelastic nature, the mode of action of the load may differ from that considered under the action of nominal tensions [1]. 93

6 b a Fig. 3. Fatigue durability in corrosive environment, respectively in the air for smooth specimens made of E36 (a), respectively E24 (b) steel brand. In Fig. 4 are shown the curves of durability in corrosive environment of welded specimens made of the two brands of steel E36 and E24. It is noted that both types of steel have similar behavior. It can be considered that this type of behavior occurs due to the similarity of the chemical compositions and the mechanical resistance characteristics. Attempts of fatigue in corrosive environment showed that specimens with stress concentrator in V have a durability 82% lower than the stress concentrator with a rounded tip. Meanwhile, tension concentrator specimens with smooth rounded tip have a durability approximately 68% lower than the smooth specimens without tension concentrator (Fig. 5). Fig. 4. Durabily at fatigue in corrosive environment for welded specimens made from E36 and E24 steels Fig. 5. Durabily at fatigue in corrosive environment for specimens without tension concentrator, with tension concentrator with a rounded tip and with a tension concentrator in V 4. CONCLUSIONS ON THE EVALUATION OF DURABILITY TO FATIGUE Comparative evaluation of the durability of smooth steel specimens made of naval steel E36 brand tested in corrosive environment highlights the fact that in the corrosive environment (3.5% NaCl solution and 6% H 2 SO 4 ) this feature is 52% less than that in air. Attempts of fatigue in corrosive environment have shown that welded specimens have less durability by about 42% than non-welded ones. 94

7 The welding process influences fatigue endurance limit, as a result of the change of the mechanical properties and the structure of the metal in the ZIT. Fatigue resistance in corrosive environment is influenced by the shape and sharpness of tension concentrator but also the level of the load that has been applied. REFERENCES [1] L. Palaghian, I.G. Bîrsan, Solicitări mecanice ale oţelurilor în medii corosive, EdituraTehnică,Bucureşti, 1999, ISBN [2] S. Băicean - Studii privind degradarea prin oboseală în mediu coroziv a unor oţeluri navale, Teza de doctorat, Galaţi, 2010 [3] C.F.Ciofu - Studies on ensuring a longer operation lifetime of the component parts of the heat exchangers - International Conference on Energy and Environment Technologies and Equipment (EEETE 14), Advances in Enviromental Technology and Biotechnology, Braşov, Romania, June 26-28, 2014 Energy, Environmental and Structural Engineering Series,nr.26, pag