welding by PRESSURE OF heavy model 34Crmo forgings

Semyonov V,¹ Jartovski, 0 ², Yrmonova³ M

¹Donbass state engineering academy

²Donbass state engineering academy

³ Priazovskiy state technical university

1.INTRODUCTION

Producing of huge welding parts is always difficulties process. It is required verge much, consumption of ener

getic and materials recourses. The aim of this work is to work out the technology, which give possibility to redirect consumption of materials and geminating of welding of construction. To fulfillment of this aim for fugue maul of this aim fur fuggier month of this aim there was working out technology of pressing welding.

On the basis of laboratory researches of the welding by pressure there was studied experimentally – industrial assay of this process at welding model of disk weighing about 11 t. There was used the bar of electro – slag melt from 34CrMo steel. The bar was forged on a diameter 880mm and cut on three parts long 570, 290 and 860mm. Joints disposed so that one of them was the in the middle collected block (in the area of maximal deformations at welding), and second — on the one-third of his length (out of this area), that identically to placing of joints at making of disk from three identical parts (fig. 1).

Figure 1: Scheme of assembling of block from three parts

2. EXPEREMENTAL PROCEDURE

The present work included the next investigations

-select of heating and pressing parameters,, studding of hardness

-studding mechanical properties of base metal and welding joint

- electronicfraktography research.

3. SELECT OF HEATING AND PRESSING PARAMETRS, STUDDING OF HARDNESS

After heat treatment the butt ends of parts were machined for welding of pressure-sealing seam. The parts metal quality was controlled ultrasonic. Pressure sealing was welded by electrodes of prelatic class using preheating up to 200-250°C. Welded block was heated up to 1200°C and pressed down the rivet by 3000 t effort press. To get a shale grain structure the block was subjected to diffusive annealing and then it is was again heated and pressed down the rivet up to required disk, sizes.

The got block was tempered and machined up to draft sizes, then its flat end were controlled by ultrasonic. Which is was not found out defects. To get the required properties a welded block was hardened by heating up to 850-870°C, cooling in to oil and stress relieved at 660°C.

The welded block investigate programmed supposed mechanical and chemical testing and also metallographic research. For this purpose from the middle part of block (fig.2) cut out 43 templates 220mm width.

Figure 2: Location of seams (1, 2) in the model of disk and its laying out on areas

At the ordinary etch in 5% HNO₃ o the lines of joints on a macrostructure did not become known. Them it was hardly succeeded to discover at the etch in a 20% - solution of HNO₃. On results, measurements the lines joints were built (fig.2). It should be noted considerable curvature of these lines, especially in a regional area: there is the sharply expressed unevenness of deformation in different parts of disk model, that most nakedly at its right edge, where relative deformation turned out almost in 4 times less, than in rests of model.

Measuring showed the considerable vibrations of hardness (НВ) in the different of model (fig. 3).

Figure 3: Distributing of hardness on the section of disk model

The maximal hardness looked after in regional areas (2090—2230 MPa) did not exceed maximum possible according to technical requirements (2500 MPa). However in central part of model hardness fell to 1540—1650 MPa, that, presumably, it is related to insufficient plenitude of hardening (by small speed of cooling of disk middle).

4. STUDDING OF THE MECHANICAL PROPERTIES OF BASE METAL AND WELDING JOINT

It led, as the tests showed, to the substantial lowering of mechanical properties of parent metal and, as a result, the welded joints in this area. Results of tests on tension at 20°C (table. 1) and 350°C (table. 2), conducted on samples cut out from the different areas of model, showed the following.

Table 1 Mechanical properties of the welded joints and parent metal of disk model at the temperature of 20°C

Disk area	Place of sample cutting	Mechanical properties of the welded joints and parent metal of disk model at the temperature of 20°C
		σs MPa	σВ MPa	δ %	ψ %	НВ MPa
Extreme	Parent metal, axial	472	653	22,8	58,9	2000-2100
	Tangential	496	673	24,2	66,3	2050-2200
	Joint 1	476	657	22,3	57,9	—
	Joint 2	471	653	23,7	58,1	2000-2100
Intermediate	Parent metal, axial	482	662	—	58,4	—
	Tangential	451	641	25,2	63,3	1700-2000
	Joint 1	363	579	17,1	50,6	—
	Joint 2	354	583	18,2	54,2	1650-1700
Central	Parent metal, axial	411	625	24,6	63,1	—
	Tangential	455	656	24,5	63,2	1700-1800
	Joint 1	375	591	22,8	59,0	—
	Joint 2	352	547	14,6	45,4	1540-1600

Table. 2 Mechanical properties of the welded joints and parent metal of disk model at the temperature of 350°C

Area of disk model	Place of sample cutting	Mechanical properties of the welded joints and parent metal of disk model at the temperature of 350°C
		σs MPa	σВ MPa	δ, %	Ψ, %
Extreme	Parent metal	327	597	21,0	59,3
	Joint 1	345	605	19,9	61,6
	Joint 2	347	601	20,8	60,3
Intermediate	Parent metal	254	549	23,3	58,6
	Joint 1	285	565	21,0	56,8
	Joint 2	293	575	23,0	58,5
Central	Parent metal	318	594	20,3	59,1
	Joint 1	334	572	24,2	57,5
	Joint 2	306	573	20,4	61,3

The strength of properties of parent metal and welded joints substantially change on the areas of model; they achieve most values at its edge and the least in a center.

Thus, there is clear correlation of these properties with hardness, indicative on decision influence of heat treatment on properties of parent metal and welded connection in the different| areas of model. The strength of properties of parent metal does not almost depend on position of samples (axial or tangential), and plastic properties of parent metal and welded joint little differ and have the least values in a central area with low hardness, that confirms their direct connection with the condition of heat treatment.

The tests on a bend conducted on sample by the size of 10x20x160 mm on mounting by a diameter 20 mm in place of 40 mm(according to requirements). On samples from a parent, metal and welded joint at angle of bend of 180° it was not discovered not only some damages, but even the local deformations related to welding. These tests confirmed exceptional homogeneity of mechanical properties of metal in the zone of connection.

Tests on a impact, most expressly reflecting quality of the welded joints, showed that strength of joints approached to the proper indexes of parent metal, except for a narrow area about 50 mm width at the right edge of seam (fig. 4). It notedly changes on the section of disk, thus most its values correspond to the zones with maximal hardness (>2000 MPa). In the fractures of sample with the lowered impact, strength it is not exposed some specific defects of joint. Pays on itself attention connection between impact strength of regional area of joint 2 with its plastic deformations. Wherein this deformation characterized by distance between seams is relatively small, there is the decline of impact strength.

Figure 4: Distributing of impact strength on the section of disk model: light circles are the maximal and minimum values for the welded joint; dark circles are the mean values for a parent metal

5. study of impact toughness of welded joints under the temperature from - 40 C⁰ up to + 40 C⁰

Thus, lowering of impact strength on the separate areas of joints is related not to the specific defects of welding (cracks|, by considerable oxides tapes), but with the features of structure of metal in the explored areas.

Both parent metal and welded joint in the range of temperatures from -40 to 40°C does not have the obviously expressed threshold of cold brittleness (fig. 5). At T<20°C impact strength goes down gradually, thus more noticeable at a parent metal.

For the estimation of propensity to fragile destruction tested samples with the initiated crack. Work of increasing of crack for the parent metal all welded joints was practically identical. In the interval of temperatures from -40 to -20°C it in both cases is small (05—1,5 J/cm²)

a- is the left extreme area (template №2, seam 1)

b- is the right intermediate area (template №31, seam 2) of Denotation is see of fig. 4.

Figure 5: Dependence of impact strength on the temperature of tests

For fatigue testing the smooth cantilever type samples from welded joints, and axial and tangential of a parent metal were used. One of welded joints was located in the critical section. Testing were done at the symmetric changeable ladening on base 10⁶ cycles (fig.6).

a- is the welded joint (seam 1, template №25);

b- is parent metal (template №39)

Figure 6: Results of tests on a fatigue

At equal hardness of critical section metal, the fatigue strength of welded joints and a parent metal did not differ practically, here with hardness increasing the fatigue strength rises appropriately.

At micro-examination of the welded samples the line of joints does not come to light. In areas with the best thermal working of metal grain (.>2000 MPa) there is a sorbite structure with the martensite orientation at a shallow corn (fig. 7, a), on areas with low hardness (1540—1660 MPa) a structure is ferrate-pearlitic (fig. 7, b). No welding defects was noticed

a - extreme area, b – central area

Figure 8: Mikrostructure of disk model welded joints (x300)

The carried out researches show that at the accepted degree of press down rivet of distinction in the conditions of deformation of central and displaced seams did not influence on quality of joints.

In connection with large influence of heat treatment conditions on quality of the welded joints separate samples with low hardness (1560-1660 MPa) exposed to the repeated heat treatment (tempering with 860°C in oil, stress relieving at 660°C), whereupon their hardness attained 2350—2500 MPa. Mechanical properties of the welded joints and parent metal considerably increase and little differed from each other.

Conclusions

1. At the auto vacuuming welding by pressure of disk model from steel of 34 CrMo equal strength welded joints and a parent metal are got, both at the symmetric location of welding plane and at its displacement, proper to the simultaneous welding of three parts equal on a height.

2. At welding it was not exposed direct influencing of sizes of parts on quality of joints, that is mean of the even heating of parts on all volume and stability of deformation condition in the process of press down rivet.

3. Mechanical tests, and also micro- and electronic fractography| researches did not found substantial distinction in a structure and properties directly in a welded joints and in parent metal.

4. Distinction in mechanical properties and structure of separate areas of disk and his welded connections is related mainly with the different terms of their heat treatment.