Influence of stainless steel circular pipes welding methods and parameters to the process accuracy and productivity/Nerudijanciojo plieno apvaliu vamzdziu virinimo metodu ir parametru itaka proceso tikslumui ir nasumui.
Bieksa, E. ; Mankute, R. ; Bargelis, A. 等
1. Introduction
Many industries are frequently use stainless steel tubular
constructions, which are manufactured using welding. While welding tube
structures is especially important to keep the elements in right
dimensions, in particular axial and angular parameters, which are
strongly influenced by welding residual stresses. They arise due to
different heating and cooling conditions of the joint components and
adjacent zones of the welded parts. That is the main reason of a
deformation the joined parts.
The stainless steel circular pipes welding is one of different
available methods often applied in industry tungsten inert gas (TIG) and
(metal inert gas) MIG are two types of often use welding methods amongst
the few choices available for this aim [1]. Laser welding techniques [2]
widely is used for thin parts or tubes and light materials magnesium and
aluminium. Welding parameters must be controlled carefully seeking
process quality and productivity. This is related with welding process
energy consumption and travel speed which should be kept uniform and as
high as possible [3]. Another way to increase productivity is to enhance
robotic welding [4]. It shows how the product modularization can reduce
the total number of parts and simplify of a product's structure.
This improved the robot welding giving benefits of manufacturing because
productivity and quality are increased. Welding process accuracy
influences on product quality when reworks of non-quality products are
avoided [5]. The destructive and non-destructive techniques to evaluate
the weld quality in modern welding production are used [6]. This
research examines destructive methods; also some attention has been paid
to find future trends in the development of new welding evaluation
approaches. The review of non-destructive methods and techniques used to
evaluate the weld quality are presented [7]. Some attention has been
paid to look future trends in the development of welding evaluation
approaches. This study provides a good foundation for learning and
creates awareness among the metal industries to evaluate their work
accuracy, quality and productivity in the field of welding.
Welding process accuracy and quality is analysed in research papers
through residual stress and fatigue strength during past years. Fatigue
strength of different notch classes regarding post weld treatment
methods and repair techniques in consideration of size effect is
examined [8]. Welded tubular steel constructions must be checked by the
critical aspect in the design of trussed-girder bridges and other
products with the local hot spot stresses at the pipe intersection. The
joint properties and their improvement in thin walled circular pipe
friction welded joint for an AISI 310S austenitic stainless steel are
considered [8]. The welded specimen with a pipe thickness of 1.50 mm was
made at a friction welding machine with pressure of 120 MPa and the
joining could be successfully achieved and that had 100% efficiency with
the base metal fracture. However, the joining became difficult with
decreasing pipe thickness, and it was not successful at a pipe thickness
of 0.50 mm [9]. It was found that when pressure in friction welding
machine is decreased to 30 MPa, the joining could be successfully
achieved, although that did not have 100% efficiency. Stress corrosion
cracking (SCC) in 316L stainless steel recirculation pipes have been
observed near butt welding joints [10]. These SCC in 316L stainless
steel grow near the welding zone mainly because of the high tensile
residual stress caused by welding. The distribution and scatter of
residual stress were measured by stress relief and X-ray diffraction
methods. The effect of welding parameters on residual stress
distribution have been evaluated through welding simulations based on
finite-element analysis using three dimensional and axisymmetric models.
The objective of this research is to examine stainless steel
circular pipes welding parameters on the process accuracy and
productivity. The interfacing of different welding methods on accuracy
and productivity were investigated applying a different thickness of
pipe walls. Experimental and analytical studies have shown stainless
steel pipe welding deformations. Manual and automatic method of TIG was
used in analysis of welded pipe shrinkage and samples' axis
deviation. Recommendations for keeping stainless steel pipes axial
position accuracy during welding and available productivity variations
are given.
2. Research methodology
In modern manufacturing environment the competitive criteria are
products and processes quality, cost and productivity. Welding processes
are urgent to use secure methods achieving product strength, reliability
and accuracy with competitive prices on the market. The welding
operations productivity plays the main influence to be competitive and
winning orders. Analysis of Lithuanian companies involved in welding
process business exposed the main interferences of work productivity,
which are: under accuracy, bad welding seams' quality, rework of
welded products, low level use of robotic in welding processes and low
skill of welding employees.
Numerous publications analysed in introduction of this paper deal
with problems mentioned above. Analysed research, unfortunately, is
related with separated problems of welding processes, as strength,
residual stresses and parameters of different welding methods. The aim
of this paper is to look interfacing of different welding parameters to
the final result of welded product and applied process to be competitive
in a market. The framework of carried out research is presented in the
Fig.1. First stage is analysis of every product's welding parts and
subassemblies looking for errors and bad manufacturability. If
available, some product's design exchange or modification must be
made. Second stage is appropriate welding process and method selection
that would be achieved required accuracy and other parameters. Chosen
welding process and method by productivity index is checked at the early
process development stage. Productivity index in this methodology is
defined as a ratio between created value and incurred cost for it. If
productivity index is not satisfied, the welding process has to be
re-developed. The final stage is divided for product total testing and
delivering to customer. Applying the developed framework the strategy
'make or buy' has to be considered looking the cheapest
possibilities of potential partners, because they often can propose
better alternative then itself producer.
For realization of developed framework three welding methods have
been used:
1) manual TIG;
2) automatic TIG;
3) automatic plasma + TIG.
The parameters of mentioned three welding methods and dimensions of
tested pipes are presented in Table 1. The quality of welding seams has
been checked applying methodology described and used in [6, 7]. The
calculation of welding time T in s has been carried out using
company's statistical data and timing procedure:
T = [T.sub.set]/p + [T.sub.grip] + [T.sub.w], (1)
where [T.sub.set] is set up time of a welding operation in s;
[T.sub.grip] is gripping time of parts in welding operation, s; p is the
batch size; [T.sub.w] is welding time, in s.
[FIGURE 1 OMITTED]
[T.sub.set] and [T.sub.grip] are defined from companies'
statistical data, while [T.sub.w] is calculated applying timing charts.
T in final stage is multiplied by overheads coefficient k = 1.1-1.25.
The incurred welding cost is defined as follows:
[C.sub.w] = W + T x A/ 3600, (2)
where W is cost of welding materials, EUR; A is welding operation
cost in EUR/h.
A = [S.summation over (l=1)][l.sub.1], (3)
where L is cost portion of technological operation in EUR/h; l is
number of variables, i.e. machine and space cost, machine maintenance
cost, internal logistic and auxiliary materials cost.
Analysing the statistical data and manual calculation methodology
in sheet metalworking companies applying welding operations, the
definition of value A has been done.
3. Experiments' results discussion
Twenty pieces of circular [theta]73 mm diameter pipe from the
standard austenitic stainless steel AISI 304L (international marking: EN
1.4307) were chosen for the experiment. Length L = 150 [+ or -] 0.05 mm
and thickness e = 3.05 mm. The chemical composition of AISI 304L is: C
0.02; N 0.04; Cr 18.2; Ni 10.1. Ten pieces of tube were welded manually
and ten pieces by automatic TIG method.
While welding pipes manually using TIG method tube edges were
prepared making 30[degrees] angle beveling (Fig. 2, a); while preparing
for manual welding pipes were tack welded in four points positioning
90[degrees] and keeping 2 mm gap between the retaining rods, which
ensures a better root penetration weld. In the order to reduce the
welding deformations [empty set]73 mm diameter pipes were welded
dividing the seam into four parts along the perimeter and welding contra
sides first (Fig. 3), then the same procedure repeated on the second
seam.
Electrode, the arc and the environment around the melted metal bath
protected from the atmosphere by argon Ar 99, 99% inert gas, which fed
into the welding zone 8-10 l/min., also to keep good quality and
geometry of welding seam argon Ar 99, 99% inert gas is supplied inside
pipe 10-12 l/min. Filler metal rods are fed to the front of the melted
metal bath [11].
The Automatic TIG pipe welding method for a special [theta]73 mm
diameter pipe does not requires edge preparation (Fig. 2, b). The pipes
at the beginning of the process are tack welded in four points every 90
degrees around the pipe circumference without gap and without the
additive wire rods. Welding seam is formed continuously in two layers
rotating pipe in electric spinner support. Electrode, arc, pipe inside,
and melted metal bath are protected from the environment by gas, which
is frequently used in stainless steel TIG welding process. Compared to
pure argon Varigon H5 gas improves weld ability and welding speed.
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
To determine the deformations occurred during welding, after each
weld both for manual and automatic welding, length of samples was
measured using altimeter while hot and at the room temperature. Length
of welded pipes samples is given in the Table 2.
Analysed data in the Table 1 shows that the manually welded samples
length after first seam and after cooling to room temperature
deformations are varied from 3.15 mm to 3.55 mm and after welding a
second seam and cooling the change of a total length varied from 3.75 mm
to 4.25 mm. Automatically welded samples length after first seam and
first cooling to room temperature deformations are changed from 0.05 mm
to 0.35 mm and after welding a second seam and cooling change of the
total length varied from 0.40 mm to 0.95 mm. It can be seen clearly that
during automatic welding sample length deformations are bigger than the
deformations after manual welding. This is explained by the fact that
during manual welding to ensure seam quality was used edge preparations
with 30[degrees] angle chamfer and samples were welded with the 2 mm
gap. Length deformation also affected by uneven seam welding speed,
which is determined by human factors. Another reason is that automatic
welding goes in series from the first point at the process beginning to
the same point at the process finishing. The welding process has been
carried out from the bottom upward.
[FIGURE 4 OMITTED]
While welding pipe structures is especially important to keep the
dimensions of the drawings, axial positions and angular parameters.
During the experiment to measure axial deviations measuring machine
"ROMER SIGMA-2022" was used. Measuring prism, presser and
measuring machine table were used for samples basing and measuring. Fig.
4 shows the principal scheme of a measuring.
For the evaluation of axial deviations welded cylindrical surface
is divided into eight parts. In the measuring device one end is based in
the prism and pressed by constant force. Measuring machine head measures
distances from the base surface (measuring machines table) in two
sections A-A and B-B in each of the eight points. To measure the
distances from cylindrical surface to the base surface of measuring
machines in all eight points sample must be rotated about the axis
45[degrees] degrees.
It was measured ten manually welded samples, and ten automatically
welded samples. Distances from a cylindrical surface to the measuring
machines base surface were measured. After the analysis of the
measurements results has been found samples' number with the most
axial deformations.
Difference in distances from the cylindrical surface to the base
surface in sections A-A and B-B (Table 3, variant W1) shows the
deviation from parallel of the base surface. A positive difference
generates leaning down from the sample axis, the negative difference
generates leaning from the top of the sample axis.
For manual welded sample most axial deviations were noticed at
second and sixth points. The biggest positive difference at sixth point
means that sample axis is leaning down. To avoid this deformation at
sixth point we must put determined heat quantity in this position.
For straightening of the sample using the TIG welding machine at
the sixths point has been put heat in 10 mm long zone. After cooling the
sample to room temperature, it was measured the distance to base surface
in sections A-A, and B-B in each of the eight points. The difference in
distances is presented in Table 3, variant W2.
[FIGURE 5 OMITTED]
After the analysis of the measurements of manual welding results,
it can be seen that at the second and sixth points still have the
largest deviations from the parallel of the base surface. Therefore, one
more time, at the sixth point heat was put in 10 mm long zone. After
sample cooling to the room temperature, distance was measured to the
base surface in sections A-A, and B-B in each of the eight points. The
difference of these measurements is presented in Table 3, variant W3.
The distribution of differences for each welding variant is presented in
Fig. 5.
After the analysing the results data in Table 3 and Fig. 5 it can
be concluded that the difference in distances from the cylindrical
surface to the base surface in sections A-A and B-B is decreasing then
putting determined heat quantity in a certain area of the welding seam.
This means that samples' axial deviation is decreasing, and the
sample straitens.
The measurements of automatically welded sample show the largest
deviations from the parallel of the base surface at the fourth and
eighth points. The biggest positive difference at eighth point means
that sample axis is leaning down. To avoid this deformation at this
point must be put a determined heat quantity in this position. For
straightening of the sample, using the TIG welding machine, at the
eighth point the heating was put in 15 mm long zone. The automatic
welding positive difference (1.19 mm) is larger than the largest
positive difference of the manual welding (0.8 mm). So the bigger
determined heat quantity (instead of 10 mm was heated 15 mm length
zone). The heating of welded samples by TIG welding device with
electrical power strength 75A and 8-9.1 V voltage in all used cases has
been carried out. After cooling the sample to room temperature, was
measured distance to base surface in sections A-A and B-B in each of the
eight points. After the analysis of the measurements of automatic
welding results, it can be seen that at the fourth and eighth points
still have the largest deviations from the parallel of the base surface.
Therefore, one more time, at the eighth point the heat put in 20 mm long
zone. After cooling the sample to room temperature, it was measured
distance to base surface in sections A-A, and B-B in each of the eight
points. After the analysing the results data in Table 3 and Fig. 5 it
can be seen that the sample straitens like in the manual welding case.
Conclusion can be made, that for straightening welded pipes samples
determined heat quantity in a certain area of the welding seam has to be
put. The heating quantity for both welding cases is defined
experimentally.
The welding process productivity with welding speed directly is
related. The appropriate tests finding an optimal welding speed have
been used. These tests are bonded with welding method, variant and type,
power strength and voltage, also pipes dimensions. After numerous
occasions changing and combining above-mentioned parameters the optimal
welding speed and process manufacturing time has been found (Table 1).
Manufacturing time is a main constituent influencing to the welding
process incurrent cost seeking the higher productivity index.
4. Implementation and further research
The developed framework and methodology seeking welding process
accuracy and productivity is implemented in Lithuanian industry. The
implementation results in some companies have shown that the developed
methodology is able to create an optimal welding process in virtual
environment and to check incurred cost [C.sub.w] (Eq. 2) of its
realization. Majority of companies, unfortunately, higher productivity
index I seek only decreasing [C.sub.w]:
I = V/[C.sub.w], (4)
where V is created value of product and process design in EUR.
Such way, however, is not best decision because created higher
value V [12] in many cases of product and process design can increase
index I much more. This problem becomes very important to Lithuanian
industry because majority of its manufacturing companies (till 80%) make
only parts and components without engineering and design. As
product's life cycle shows the additional value of new product and
process at the early design stage is created. Many social and domain
factors influence to employees creativity and work motivation getting
better experience and cleverness of innovative ideas generation and
implementation in design process. It often requires changing the former
order of organization activity preparing and sharing the information for
the integrated product and process design. Some times it causes the
complaints of the employees in an organization. There are many methods
and possibilities how to solve mentioned problems: getting consultations
from social and domain experts, implementing long life learning (LLL)
methods in organization and searching chances for better corporal social
responsibility (CSR) to employees.
It is planned in further research activity to develop an interface
of creativity improvement in general framework of innovative product and
process design. The appropriate methodology has to be foreseen involving
and implementing an Internet technology and various web sites employing
knowledge base (KB) and intelligent methods in manufacturing engineering
and new products design. The value engineering method is one of more
important fields in developed methodology. It includes the value
estimating and creating chapters both for total product and process and
their separate parts and components.
5. Conclusions
1. After the analysis of welded pipe samples, it can be seen that
during automatic welding length deformation is smaller than the length
deformation after manual welding. This can be explained by the fact that
during manual welding to ensure seam quality it was used the pipe edge
preparation with 30[degrees] angle chamfer and samples were welded with
the 2 mm gap. Automatic welding does not require any gaps between pipes.
Length deformation during manual welding also affected by uneven seam
welding speed, which is influenced by human factors.
2. After the analyzing the sample axial deviations it is
determined, that deviations after TIG automatic welding are bigger till
0.2 mm then after manual welding.
3. Both automatically and manually welded samples after putting
determined heat quantity in a certain area of the welding seam
straightening simultaneously.
4. The optimal welding speed has been found experimentally changing
welding method and type, power strength and voltage for higher
productivity index.
Acknowledgement
This research was partially supported by EC project grant agreement
No 2007-1990/001 "Inter countries research for manufacturing
advancement (IRMA)" in period from 01. 01. 2008 to 31. 12. 2009.
http://dx.doi.org/ 10.5755/j01.mech.19.2.4161
Received April 06, 2012 Accepted April 08, 2013
References
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E. Bieksa *, R. Mankute **, A. Bargelis ***
* Kaunas University of Technology, Kcstucio 27, 44312 Kaunas,
Lithuania, E-mail: egidijus.bieksa@stud.ktu.lt
** Kaunas University of Technology, Kcstucio 27, 44312 Kaunas,
Lithuania, E-mail: rasa.mankute@ktu.lt
*** Kaunas University of Technology, Kcstucio 27, 44312 Kaunas,
Lithuania, E-mail: algirdas.bargelis@ktu.lt
Table 1
Welding processes methods and main parameters
Process Current Voltage, Number of
Welding method code power V welding
strength, passes
A
Manual 141 60-65 8-9 2
TIG 70-75
141 65-70 8-11 3
75-80
75-80
141 130-150 10-15 4
135-160
160-180
160-180
Automatic 141 75-80 12-14 2
TIG 75-80
Automatic 15 210 30 3
Plazma +TIG 141 215-230 13-15
141 215-230 13-15
15 240 34 4
141 250-270 16-18
141 260-280 16-18
141 280-300 16-18
Pipe wall External Wel- Total
Welding method thick- diameter, ding welding
ness, mm mm time, time, s
s
Manual 3.05 [empty set]73.1 344 386
TIG
5.16 [empty set]73.1 517 562
8.18 [empty set]219.1 2064 2157
Automatic 3.05 [empty set]73.1 230 348
TIG
Automatic 5.16 [empty set]73.1 264 383
Plazma +TIG
8.18 [empty set]219.1 1116 1603
Table 2
General information of considered welding methods
Manual welding
Sample Length after Length after
length first seam, mm second seam, mm
No. after Hot Cold Hot Cold
tack
welding,
mm
1 301.90 299.05 298.65 298.60 298.15
2 302.10 299.30 298.95 298.70 298.20
3 301.85 299.00 298.55 298.35 297.95
4 301.85 298.95 298.40 298.15 297.80
5 302.05 299.15 298.65 298.40 298.00
6 301.95 299.00 298.75 298.55 297.95
7 301.90 298.90 298.35 298.05 297.75
8 302.00 299.05 298.65 298.35 297.90
9 302.05 299.25 298.70 298.25 297.80
10 301.90 298.85 298.35 298.00 297.65
Automatic welding
Sample Length after Length after
length first seam, mm second seam, mm
No. after Hot Cold Hot Cold
tack
welding,
mm
1 300.05 300.10 299.95 300.00 299.65
2 300.05 300.15 299.85 299.75 299.15
3 300.10 300.35 300.05 300.00 299.15
4 300.00 300.05 299.65 299.65 299.30
5 300.05 300.15 299.80 299.85 299.45
6 300.10 300.10 299.90 299.85 299.55
7 300.00 300.05 299.75 299.75 299.35
8 300.10 300.10 299.85 299.90 299.40
9 300.00 300.05 299.70 299.80 299.45
10 300.10 300.10 299.85 299.85 299.50
Table 3
Differences in distances from the cylindrical
surface to the base surface in sections A-A and B-B
Manual welding Automatic welding
Points Welding variants Points Welding variants
W1 W2 W3 W1 W2 W3
P1 -0.71 -0.39 -0.16 P1 0.46 0.30 0.23
P2 -0.83 -0.64 -0.38 P2 -0.46 -0.37 -0.31
P3 -0.62 -0.14 -0.13 P3 -1.02 -0.88 -0.49
P4 -0.05 0.19 0.04 P4 -1.2 -0.87 -0.47
P5 0.35 0.18 0.18 P5 -0.48 -0.12 -0.12
P6 0.58 0.39 0.19 P6 0.62 0.59 0.39
P7 0.49 0.29 0.16 P7 1.11 1.03 0.57
P8 0.14 0.16 0.15 P8 1.19 1.07 0.60
Welding variants: W1--primary welding; W2--welding
after 1st heating; W3--welding after 2nd heating.