Ultrasonic Impact Treatment. Physics of the Process. Application. Results design features equipment. E. S. Statnikov

Ultrasonic Impact Treatment.

Physics of the Process. Application. Results. design features. equipment.

E.S. Statnikov

Applied Ultrasonics, P.O. Box 100422, Birmingham, Alabama, 35210, USA

Northern Scientific and Technology Company

6 Voronin St., Severodvinsk, Arkhangelsk Region, 164500, Russia

Abstract

Considerable recent attention has been given to post-weld and concurrent treatments of welded joints. These treatment methods are intended to increase the quality, reliability and life of welded structures. Ultrasonic impact treatment (UIT) has a particular place among them.

Independent expert assessments of this method obtained from the laboratories in Russia, Ukraine, France, Norway, Sweden, USA and Japan pointed out the effectiveness of this technology.

The paper covers the information on the design features of the UIT process systems, physics of UIT, equipment and areas of application, and UIT effect on the fatigue resistance.

1. 
   introduction
   

The paper presents the results of the development [1] and study of UIT benefits for increasing the fatigue limit of welded joints. The UIT technology and equipment have been developed at NSTC (Severodvinsk, Russia). At different times, the UIT method was adapted and studied at the E.O. Paton Electric Welding Institute, Ukraine together with NSTC and based on the orders of NSTC. The results of the UIT study performed at the Norwegian University of Science and Technology, Trondheim, Norway and SSAB, Sweden are presented.

2. 
   method for ultrasonic impact treatment of welded joints [2]
   

The method is based on the instrumental conversion of harmonic oscillations of the ultrasonic transducer into peening impulses of ultrasonic frequency and their further conversion into harmonic oscillations of a treated surface and a structure element at an excitation frequency.

In the course of the method development we achieved the optimization of an active element being a part of the ultrasonic tool which allowed us for minimization of sizes and 4 times increase in specific power as compared to the conventional manual tools [3].

The UIT zones of action are shown in Fig. 1 in a cross-sectional view of the surface layer of the welded joint metal being treated.

Zones	EFFECT
"White layer"	Wear-resistance, corrosion resistance
Plastic deformation	Cyclic endurance, compensation of deformation, corrosion-fatigue strength
Impulse relaxation	Reduction in residual welded stress and strain of up to 70% of the initial state
Ultrasound relaxation	Reduction in residual welded stress and strain of up to 50% of the initial state

Fig. 1 UIT action physical zones

Figure 1 and the accompanying table represent the UIT study results. The results define the topical areas of UIT application in production, maintenance and repair of welded structures. Only one area of application is addressed below, namely the fatigue resistance of the welded joint metal.

3. 
   UIT effect on fatigue strength of welded steel joints [4]
   

Presented here are the fatigue test results for welded joints from steel WELDOX 420 made at SSAB, Sweden. The results were obtained at the E.O. Paton Electric Welding Institute of the Nas of Ukraine in cooperation with NSTC and to the order of NSTC. The intention was to compare the effectiveness of UIT with that of well-known methods of shot peening, hammer peening and TIG dressing of weld toes. The tests were in accordance with IIW Collaborative Test Program on Improvement Methods (Doc. XIII-WG2-30-94).

3.1. Material and Specimen Fabrication

Specimens were fabricated from WELDOX 420 steel 20mm in thickness made at SSAB, Sweden. Mechanical properties of the steel as determined during study are given in Table 1.

Table 1 Mechanical properties of steel

Specimen No.	Yield strength, MPa	Ultimate tensile strength, MPa	Elongation, %
1	457,1	571,9	28,0
2	463,7	571,5	28,8
3	462,7	575,5	29,6
Average	461,2	573,0	28,8

Plates for specimen fabrication were welded at SSAB by the SAW process. Esab welding materials were used: OK 12.32 / OK Flux 10.71.

The welding conditions were as follows:

Interpass temperature: 150 – 170ºC, DC+
Pass No. 1: side 1, 925A, 34V, speed = 39 cm/min ; HI = 4,8 kJ/mm
Pass No. 2: side 1, 720A, 30V, speed = 49 cm/min; HI = 5,8 kJ/mm
Pass No. 3: side 1, 720A, 30V, speed = 49 cm/min; HI = 5,8 kJ/mm
Pass No. 4: side 2, 925A, 34V, speed = 39 cm/min; HI = 4,8 kJ/mm
Pass No. 5: side 2, 720A, 30V, speed = 39 cm/min; HI = 4,8 kJ/mm
Pass No. 6: side 2, 720A, 30V, speed = 39 cm/min; HI = 4,8 kJ/mm

A
total of three plates were welded to the sizes shown in Fig. 2.

1 – as-welded specimens; 2 – UIT treated specimens; 3 – hammer peened specimens; 4 – shot peened specimens; 5 – TIG dressed specimens; 6 – specimens subjected to TIG dressing followed by UIT;

Fig. 2 Geometry of welded plates for fabrication of as-welded and improved specimens

The plates were cold sawn into specimen widths and then milled to finished sizes, as shown in Fig. 3. 54 fatigue test specimens were made in the as-welded condition (1 series) and improved by various treatments (5 series). Each series was made up of 9 specimens.

F
ig. 3 Fatigue test specimen geometry

2. 
   Fatigue Test Results
   

Based on the results, the benefits of hammer peening, shot peening, TIG dressing and combined treatment of TIG dressing and UIT were virtually the same. These improvement treatments increased the fatigue strength at 2 x 10 ⁶ cycles by 41-51%.

The considerable improvement, 65%, was obtained from the UIT treated joints with treatment performed using the 3mm multi-pin tool. Comparison of fatigue curves for the as-welded joints and welded joints treated by UIT using various parameters (including 5mm diameter pins) shows that the maximum effect is obtained by treatment with a 3mm multi-pin tool which produces a groove (up to 0,8mm in this particular case) at the weld toe.

2. 
   Conclusion
   

Comparison of all the fatigue curves obtained, shown in Fig. 4, indicates that UIT gives the great benefit as against other improvement techniques. It should be noted that the benefit from UIT depends on the treatment parameters.

Linear regression :

Series 1 – as welded (m = -5,6867);

Series 2 – UIT treated (m = -17,5413), 5 and 3mm pin diameter;

Series 3 – hammer peened (m = -8,3701);

Series 4 – shot peened (m = -8,1321);

Series 5 – TIG dressed (m = -6,5539);

Series 6 – TIG dressed and UIT treated (m = -6,2090);

Series 7 – UIT treated (m = -9,7799), additional series, 3mm pin diameter.

Series No.	Results of approximation	Δ (N=2000000 cycles), MPa	After treatment increase of Δ, %
1	lgN=19,3650-5,6867*lg	198,2789	-
2	lgN=50,4231-17,5413*lg	327,5881	65
3	lgN=26,7702-8,3701*lg	278,9457	41
4	lgN=26,2096-8,1321*lg	280,6418	42
5	lgN=22,4068-6,5539*lg	286,7175	44
6	lgN=19,3650-6,2090*lg	293,2463	51
7	lgN=31,0829-9,7799*lg	341,9457	73

Fig. 4 Fatigue curves for WELDOX 420 welded joint in the as-welded and improved conditions

4. 
   UIT effect on fatigue strength of welded aluminum plates [5]
   

Two types of aluminum specimens were tested in the as-welded condition and improved by ultrasonic impact treatment at the Norwegian University of Science and Technology, Trondheim, Norway in collaboration with SSAB, Sweden.

4.1. Test Procedure

The aluminum specimens were fabricated from a typical ship plate material, AA5083 (or AlMg4.5Mn). The minimum specified yield and ultimate tensile strengths were 250 and 335 MPa, respectively, with a ductility of A5 = 10%.

Two basic types of aluminum specimens were tested. One is a lap type specimen shown in Fig. 5.

Fig. 5 Aluminum lap joint fatigue test specimen

The second type of aluminum specimen is shown in Fig. 6.

Fig. 6 Aluminum specimen with longitudinal attachments

The specimens were fillet welded using standard shop practice at Kværner Fjellstrand shipyard. The UIT was performed at Kvant in Severodvinsk according to standard practice. The aluminum lap joints had good quality welds. The aluminum specimens with longitudinal attachments, however, were weld at high production rates and had welds of poor quality at the ends of the attachments. Ultrasonic Impact Treatment, however, improved the shape of the end of the weld considerably.

All specimens were tested in axial loading, constant amplitude at R = 0.1 The test frequency was in the range 3 to 10 Hz. Failure was defined to have taken place upon complete separation of the specimen.

4.2. Results and discussion

T

he test results for as-welded specimens with longitudinal attachments and those improved by UIT are plotted in Fig. 7. Although the untreated weld had a very unfavorable shape at the stiffener ends the S-N curve for improved weld is quite high. Two of the specimens treated by UIT failed outside the weld, the cracks initiated at small defects on the plate. The improvement in fatigue strength at two million cycles is more than 90%, corresponding to a factor on life of 20 over the entire range of the data.

R =0.1

Fig. 7 Results on aluminum specimens with longitudinal attachments in the as-welded and UIT improved conditions.

The test results for aluminum lap joints are presented in Fig. 8. These joints were tested in the as-welded and UIT improved condition.

 As-welded

 UIT

Fig. 8 S-N data for aluminum lap joints in the as-welded condition and UIT improved

The improvement due to UIT is somewhat lower than for the aluminum specimens with longitudinal attachments. The reason is probably that the as-weld S-N curve is quite high so further improvement is not possible. This assumption is substantiated by the fact that several failures initiated on the plate.

4.3. Conclusions

UIT gives consistently large increases in fatigue strength, particularly at long lives, for welded AA5083 aluminum plates.

The improvements in fatigue strength at long lives due to UIT are 80 %.

5. 
   Design of UIT Process Systems [6]
   

The traditional approach to the design of the ultrasonic process systems more often assumes the qualitative determination of the technical requirements to the selection of the oscillating system type, frequency and power of the ultrasonic transducer and generator.

In this case, the matching with the load, as a rule, is accomplished in the final stage of the selection and adjustment of the generator components, and the ultrasonic transducer is taken as a load. More rarely, the load includes the process object, i.e. the object acted upon by the ultrasonic oscillating system.

At the same time, the object’s response to the ultrasonic oscillating system under high dynamic and static forces, imparted to this system during the action, is the main indicator of the effectiveness of the operational process, as well as of the optimum matching and adjustment in the system object–transducer–generator. Here, by the object’s response to the action is meant, on the one hand, the oscillations of the object and on the other the structural changes in properties and stressed state of the object material.

From this follows the development logic of ultrasonic specific-purpose systems.

Conclusion

During the development of the ultrasonic process systems, along with the traditional approach to the selection of the oscillating system and generator parameters, it is prudent to sequentially use the acoustic study results of the process load with the conversion of this load into the form of the acouso-mechanic load applied to the ultrasonic transducer, and then conversion of these loads into the form of the electric load applied to the generator.

6. 
   UIT equipment [7]
   

UIT of welded joints is performed with the equipment consisting of an ultrasonic tool with operating frequency of 27, 36, 44 kHz and the associated ultrasonic generator (Fig. 9).





Fig. 9 UIT equipment

5.1. Specifications of Ultrasonic Tools

Parameter	Operating frequency, kHz
	27*		36*		44*
Design	For manual treatment For automatic treatment
Rated consumed power, VA	600-1200		300-800		200-500
Excitation voltage, V	60-110
Bias current, А	10-15		6-10		5-8
Oscillating amplitude of the waveguide output end, micron	35-40		30-35		25-30
Treatment speed in manual mode, m/min (m/h)	0,3 – 1,5 (18 – 90)
Treatment speed in automatic and semi-automatic mode, m/h	3 – 30
Overall dimensions of the manual tool, mm	455х85х80	380х110х50		330х100х40
Manual tool weight, kg	3,5	1,2		0,75
Cooling	Liquid
Replaceable tool heads	Straight, angle
Indenter diameter, mm	2-5
Hardness of the indenter work face	62-64 HRC

*Note:

27 kHz – production unit;

36 и 44 kHz – prototypes.

Manual ultrasonic tools installed on the welding machine travelers can be usable for automatic UIT. Special tools can be designed based on the standardized ultrasonic transducers developed and manufactured by NSTC (Fig. 9).

5.2. Specifications of Ultrasonic Generator UIP MSP-5

Output power Output power adjustment Supply main voltage Supply main frequency Max output voltage Operating frequency range Overall dimensions Weight:   generator   power supply Max cable length to connect generator and tool Cooling Automatic frequency adjustment Mechanical resonance indication Relative measurement of displacement amplitude of the waveguide output end

600 – 1800 VA stepped (4 ranges, 6 steps in each) 110/220 V 60/50 Hz 100 V 25 – 28,0 kHz 520 x 240x 470 mm 12,5 kg 19 kg 80 m air over the entire range light needle indicator

7. 
   References
   

1. 
   Statnikov E S: Development and study of ultrasonic specific-purpose devices. Thesis. Academy of Science of the USSR. Academic N.N Andreyev Acoustic Institute, No. A-266, 1982.
   
2. 
   Statnikov E S: ‘Increasing the life of welded joints by UIT’. Welding Conference LUT join’99. Int Conf on Efficient Welding in Industrial Applications (ICEWIA), Lappeenranta, Acta Universitatis Lappeenrantaensis 85, 1999.
   
3. 
   Statnikov E S, Kazantsev V F: ‘Design technological line of power acoustic systems for technological purposes’. World Congress on Ultrasonics, Berlin, 1995.
   
4. 
   Statnikov E S et al: 'Comparison of ultrasonic impact treatment (UIT) and other fatigue life improvement methods'. IIW Doc. XIII-1817-00, Florence, 2000.
   
5. 
   Haagensen P J, Statnikov E S and L Lopez-Martinez: 'Introductory fatigue tests on welded joints in high strength steel and aluminium improved by various methods including ultrasonic impact treatment (UIT)'. IIW Doc. XIII-1748-98, Hamburg, 1998.
   
6. 
   Statnikov E S: ‘Ultrasonic process systems for treating materials under high dynamic and static forces imparted to oscillating system’. Int Conf Ultrasonic Technological Processes-2000, Arkhangelsk, 2000.
   
7. 
   Statnikov E S: 'Guide for application of ultrasonic impact treatment improving fatigue life of welded structures’. IIW XIII-1757-99, Lisbon, 1999.