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PRACTICAL WELDING LETTER, Issue #013-- Laser Beam Welding,Thick Pipe,High Temp. Brazing,Bend Tests September 01, 2004 |
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| We hope you will find this Letter interesting and useful. Let us know what you think of it. Practical Issues, Creative Solutions
This publication brings to the readers practical answers to welding problems in an informal setting designed to be helpful and informative. We actively seek feedback to make it ever more useful and up to date. We encourage you to comment and to contribute your experience, if you think it may be useful to your fellow readers.
You are urged to pass-along this publication to your
Date: September 2004 - Practical Welding Letter - Issue No. 13
-----------------TABLE of CONTENTS---------------
1 - Introduction 2 - Article: Laser Beam Welding 3 - How to do it well: Welding of thick Pipe 4 - Filler Metal for High Temperature Brazing 5 - Online Press: recent Welding related Articles 6 - Terms and Definitions Reminder 7 - Article: Bend Tests 8 - Site Updating 9 - Short Items 10 - Explorations: beyond the Welder 11 - Contribution: Your Experience 12 - Testimonials 13 - Correspondence: a few Comments 14 - Bulletin Board
1 - Introduction. One year already passed since the first issue of Practical Welding Letter was released. It is therefore fit to pause and to sum up the work performed during this time. It was a pleasure but also hard work to keep the schedule. Unfortunately it is not clear how useful is this publication to the readers who only occasionally take the initiative to comment and to suggest improvements. And although many of the correspondents performed difficult repair welding operations on their hardware, none has as yet presented his/her achievements for the benefit of fellow welders. You are all invited to share your experience on the pages of this publication. A short article on Laser as used for welding is presented here, with basic information for all those considering the use of the tool for their purposes. It may be a solution for difficult cases. A practical approach is suggested for those confronted with welding thick wall pipes with double-V-groove preparation. This kind of thought may not be self evident when first dealing with the issues involved. This time the Filler Metal section presents high temperature brazing alloys. Important fabrications, typically in gas turbine engines, use techniques and materials reported here. Another Article shows the many uses of Bend Tests to assess the ductility and the quality of weldments. Their application should not be dismissed as unimportant. On the contrary it is recommended to apply these tests whenever possible for judging on the adequacy of production. For Site Updating we cannot report on progress besides minor adjustments, as new pages are not yet ready for publication: we hope to be able to present some new topics in the near future. The other Departments appear as usual, hopefully to provide useful insight to our readers. Your feedback is always welcome. Click here. 2 - Article: Laser Beam Welding Laser stands for "Light Amplification (by) Stimulated Emission (of) Radiation". Laser Beam Welding (LBW) uses a highly concentrated coherent (in phase) optical energy source of light to produce heat. The energy density available is about 105 to 107 Watt per square centimeter as laser beams can be concentrated to a small spot. Many unique advantages make LBW attractive for different welding operations.
Laser output, a function of power, is generally pulsed for solid state sources (Ruby, Nd:glass, Nd:YAG) (Nd = Neodymium, YAG = Yttrium Aluminum Garnet) of limited power peak. Pulsed lasers permit application of microwelds to precise and delicate items. More powerful gas (CO2) sources deliver continuous wave (CW). Maximum thickness weldable in stainless steel type 304 is up to 0.6 mm for pulsed and up to 50 mm for continuous laser, but limitations are continuously being exceeded by modern developments. For deep penetration weld, a keyhole is formed at the point of laser impingement, where the material is vaporized along all the depth and metal is melted to liquid state, moving continuously from the leading to the trailing edge and leaving a solid weld in its wake. This is a very effective way of distributing energy throughout the depth, whereas in more conventional welding processes heat is deposited at the surface of the workpiece and brought to the interior by conduction. The beam diameter presents a gaussian distribution of energy with the peak at its center. The efficiency of Laser Beam Welding depends on the absorption of light energy by the workpiece: different materials have different capacity of light energy absorption. Most solid state lasers are used for microwelding applications. Typical data for Nd:YAG lasers are as follows:
These lasers in form of rods are activated ("pumped") by flashing xenon lamps arranged in an elliptical cavity with at least two lamps in parallel. Other lasers have the form of crystal slabs. More recently emerged solid state multi-kilowatt lasers capable to provide continuous wave output. Energy is transmitted through fiber optics, with practical advantages over previous systems. Continuous wave gas lasers are generated in more powerful and complex equipment, used for penetration welding. Different configurations exist, including means to move the gas through the resonant cavity and to cool it in special heat exchangers. Work with lasers present more and different hazards not readily apparent even to welders experienced in more conventional welding techniques. Therefore it is imperative to learn and apply the requirements of the American National Standards Institute specification ANSI Z136.1, "Safe Use of Lasers", even before introducing the equipment in the shop. 3 - How to do it well: Double V-Groove welding of thick Pipe Q: A double-V-groove joint in a thick Pipe requires welding both from inside and from outside. Is there a preference as to which side to weld first? A: Yes, definitely. As the pipe is thick, the process used will most probably be either Gas Metal Arc Welding or Submerged Arc Welding which are both providing high weld deposition rate. In order to guarantee the highest quality the root pass, however being done, has to be back gouged, that is ground for all its length until sound metal is found. This grinding operation with a portable grinder can best be performed unhindered from the outside. Therefore the root pass has to be laid down first from the inside of the pipe. Additional tips: please note that run out tabs have to be provided at both ends. Furthermore the first weld (from the inside as explained) will start at one end for a length of about 150 mm (6 inches) only. This short weld will prevent distortion and overlapping of the edges under the shrinkage strains caused by the long weld. Then its inner end has to be ground clean to permit perfect blending with the upcoming weld that will start at the other end of the pipe and proceed towards the already welded short stretch. Once the root weld is completed and backgouged the filler passes will be performed as convenient. 4 - Filler Metal for High Temperature Brazing The interest in High Temperature Brazing stems from the advantages obtained by combining ease of performance (simultaneous brazing of many joints of heat resisting alloys in a single setup, usually in a high vacuum high temperature cold wall brazing furnace), with the heat resistant qualities obtainable by selecting filler metals suited to the application. The uses considered by this article involve the alloys characterized by a melting range higher than about 980 C (1800 F) which is much higher than what the more common silver base alloys can stand without melting away. The design of the joint should be selected keeping in mind the special process requirements and the capillary gap available at brazing temperature. The time at temperature should be limited to the minimum needed to bring all components at heat, and for the molten alloy to flow and fill the gap. In particular this process can be applied for joining Nickel- or Cobalt-based alloys, also described as Superalloys. Gold based and palladium-nickel based high temperature brazing alloys are also available for elevated temperature applications. Although many alloys were classified by the American Welding Society in ANSI/AWS A5.8 - "Specification for Filler Metals for Brazing and Braze- Welding", there are still many proprietary or commercially available alloys not yet classified which are nonetheless usable for practical applications. In certain instances the temperature of remelting of filler metals having undergone diffusion is higher than that used for brazing, thus permitting a gain in the high temperature applicability of this process. This is particularly evident for those alloys containing boron or silicon as melting point depressants. Although the additional elements might produce intermetallic brittle phases with metals like chromium and molybdenum, further diffusion treatments may eliminate them, by diffusing them away from the joint. By recalling the basic definition of brazing as a process capable of obtaining coalescence or joining by the use of a filler metal whose melting temperature is lower than that of the base materials, one can appreciate the usefulness of those special alloys that present high strength and load carrying capacity at high temperatures. In the following Table I a few of these brazing alloys for elevated temperature use are reported. The form of the brazing alloy should be carefully selected for the application: foil, wire, tape, paste and powder are generally available. Powder mixed with a suitable binder is often supplied in practical dispensers for hand or machine deposition. Preparation for welding, including thorough cleaning (chemical, mechanical or sometimes complex stages at heat with hydrogen fluoride gas) and proper fixturing, is a labor intensive operation. Flash Nickel plating of base metals can be used to ease wetting and speed up brazing, particularly of nickel-base alloys containing high percentages of aluminum and titanium (such as Inconel 718). All the normal process control procedures should be applied, including metallographic examination of sections of typical test piece joints or scrap parts. Visual and non destructive inspection are also required. Some of the products, notably those used for aerospace, are also covered by SAE/AMS Specifications, as indicated. It is understood that the brazing range is somewhat higher than the Liquidus temperature, with limits best being established by practical testing.
Notes:
5 - Online Press: recent Welding related Articles
Weld Repair Saves Airline Millions Innovative Repair in a Nuclear Plant Infrared Thermography Shipyard’s Welding Operations A list of recent articles is available from Note: many of these articles are free, for subscribers at no cost.
6 - Terms and Definitions Reminder
Abrasive Waterjet Machining is a cutting process using a high speed water
jet with or without abrasive particles to produce cuts into solid metals,
ceramic, glass and composite materials.
Brittleness is the tendency of a material to fracture without prior
noticeable plastic deformation. May be not an intrinsic property, but due to
stressing conditions (i. e. low temperature).
Decarburization is the loss or removal of surface carbon from a steel
body, due to interaction with an external medium in presence of contributing
conditions. (Example: heating in air). May bring unwanted properties
deterioration.
Extrusion is a production process for metals and plastics where material
is forced by pressure to flow plastically through a die containing a profiled
hole that imparts its uniform cross section to the material.
Machining Allowance is the excess material intentionally left on the
surface of a part, to control the properties of the final product after
carefully removing damaged external layers.
Phase Diagram is a graph representing the zones of existence of different
phases in an alloy as a function of composition and temperature. The graph at
equilibrium may be different but similar to that at not so stable conditions.
Powder Metallurgy consists in the technology used to form solid metal
parts by compressing blended powders in shaping dies, and then to consolidate
them by sintering at elevated temperature in special furnaces under proper
atmosphere.
Vacuum Induction Melting is a process used to refine a metal by removing
dissolved gases and impurities and to improve its properties. The process may be
repeated with different equipment and technology to achieve exceptional quality.
7 - Article: Bend Tests
Bend tests are practical means of assessing the ductility and the soundness of a
material or of a weld without using instrumented equipment, except possibly for
simple means of measuring an angle.
Depending on specification requirements, ductility is indicated by the
extent of plastic deformation in tension experienced by the outer fibers of the
bend specimen at rupture, or at the end of the required bending, if the specimen
does not rupture. If it did not crack it passed the bending test.
Brittle fracture occurs if the ductility available in the material
exposed to bend test is exceeded. Lack of soundness is visible if the fracture
follows a path of internal discontinuities opening up cracks, porosity or
incomplete fusion.
Two elements are defined for a bend test of a given specimen of known material,
condition and thickness: the radius of the mandrel around which the
specimen is made to bend, and the angle through which the bending must be
performed before fracturing will appear.
Two kinds of tests are applied. A free bend is performed using a jack or
a tensile testing machine. As an example, annealed materials at maximum softness
(like aluminum or copper) (dead soft) will usually withstand, without cracking
or fracturing, bending at radius zero through 180 degrees.
For heavy plates the free face bend is performed in two stages: in the first, an
initial bend is introduced in the specimen, supported by given rollers at a
certain distance from one another, using a special plunger specified in
applicable documents. The final free bend will be performed by applying forces
to the ends of the specimen to further bend the specimen on itself. The test
shall be continued until the appearance of visible cracks.
Guided bend test is performed using a fixture that presses the metal
against the mandrel (plunger in a press setup) or wraps the specimen around it.
Less and less ductile materials will be acceptable by code or standard if the
radius is a certain factor multiplied by the specimen thickness, and if the
angle obtainable before fracturing or cracking is as specified.
In any case, for weld testing, it is important to specify location and
direction of the weld with reference to the bend radius. In case of a groove
weld, specimens of given thickness to be submitted to bend testing may be sliced
from the weld test panel.
A root bend is done so that the maximum extension of the fibers of the
specimen is imposed upon the weld root: in this case the weld face is compressed
against the mandrel. Usually the weld reinforcement is machined flush with the
surface from both sides (face and root)
In face bend the maximum extension for a specimen as above, the weld face
is pulled in tension and the root is pressed against the mandrel. One variant of
this test, called a nick-break test, is intended to test the soundness of
a weld. Two saw cuts shall be machined, for a depth of about 6 mm (1/4") at both
ends of the specimen along the whole section of the weld before bending. The
presence of internal defects will produce rupture, exposing those defects.
Side bend is performed, when required, by laying the weld section flat
under the plunger and by deforming the whole weld thickness.
Besides for plates, bend tests can be performed also for pipes on slightly
curved surfaces.
These tests are obviously destructive and therefore they are performed on
representative test specimens. Even if there are no formal requirements of Codes
or Specifications, it is highly recommended to perform some sort of bend test to
get a feeling of production quality and to gain the chance of improving one's
dexterity and skill.
8 - Site Updating
This month no new pages were added unfortunately, because they are not yet ready
for publication. But new additions will be made as soon as possible.
Please consult the Site Map from time to time. New pages will be made evident to
help readers to consider if they are interested in reading them.
Only minor corrections were introduced, not changing substantially the content
of the Site.
If you have requirements or preferences as to the subjects to be treated either
in the Site or in this Practical Welding Letter, please let us know. Click here.
9 - Short Items
9.1 - Physical Vapor Deposition consists in coating processes with
selected atoms or molecules transferred and deposited, normally under vacuum, on parts substrates from a
source, excited by heat or by gas ions accelerated by high voltage. The
processes are versatile in that they permit coating with different classes of
materials (metals, alloys, semiconductors, superconductors and polymers) or
fabricating composites of various types.
The simultaneous deposition of multiple compounds as it were atom by atom,
depends on controlling parameters and techniques in order to obtain the desired
composition and properties of the selected coatings.
The actual equipment used is quite complex so that these processes, of which
different types and variants exist, are used for very special applications where
the performance required is more important than the costs incurred.
9.2 - Welding of Plastics is commonly performed on thermoplastic
materials, which are those reaching a mushy state upon heating but recovering
their solid consistence upon cooling down to room temperature.
Polyethylene, Styrene copolymer, Polyvinyl chloride, and Polypropylene are
typical of materials susceptible of Thermobonding or welding. Other plastic
materials which are unstable at temperature such as nitrocellulose or those
which contain components which volatilize under heat cannot be welded.
The same is for all classes of thermosetting plastics. A simple test can be done
using a rod heated to about 260 oC (500 oF) by pressing it
on plastic materials for quickly sorting them. If the material softens, it is
thermoplastic and suitable for welding. If it remains hard it is thermosetting.
Welding is performed by appropriate supply of heat with a welding gun with
controlled low temperature or with a heated soldering iron. Filler material may
or may not be used as fit for the joint type and the application. Properties
should be tested on specimens to determine the suitability of the process to the
intended application.
9.3 - HVOF - High Velocity Oxygen Fuel is a thermal spray deposition
method with powder fed into a supersonic velocity combustion flame. It operates
continuously using higher oxidant and fuel gas pressures than plasma spray,
resulting in higher particle impact velocities, thus producing extremely dense,
well bonded thermal spray coatings.
Flame temperature is around 2600 oC (4710 oF) and particle
impact velocity is 900 m/s (2950 ft/s). Elevated spray rates can be obtained,
reaching up to 15 kg/hr (33 lbs/hr). Provisions should be implemented to cool
the object being coated if the applied heat risks to damage it.
Typical material used for demanding applications is a powder consisting in a
mixture of Tungsten Carbides in a matrix of Cobalt (WC-Co).
9.4 - Infrared Temperature Sensors are noncontact devices which depend on
the thermally generated radiation from the surface of the test object. At
moderate temperatures, this energy is predominantly in the infrared region.
Therefore, noncontact measurements in thermal inspection primarily involve the
use of infrared sensors.
Usually the sensors are part of a system displaying an actual image of the
infrared picture, with good quality image resolution. The time response should
be proportionate to the speed of temperature change occurring in the test setup.
A color scale is correlated to the actual temperature ranges to show the
temperature distribution within the image.
As no contact is needed, this method is applicable to dynamic processes or to
moving machinery (typically gas turbines) through suitable quartz windows, to
verify the evolution of operating conditions.
See an Article on Infrared Thermography, as listed in the Department on Online
Press (5).
9.5 - High Pressure Gas Quenching systems are used in vacuum furnaces
if rapid quenching is required for the heated parts. Originally gas quenching
was performed at pressure lower than atmospheric, not to break completely the
vacuum. Used mostly for High Alloy and Tool Steels.
However it was found that if the equipment (originally built to stand only
vacuum against external atmospheric pressure) was made capable of accepting
inside high pressure high velocity gas circulating in the furnace, after being
cooled in a heat exchanger, the rate of cooling of parts could be increased
significantly.
Argon, Nitrogen and Helium are currently used. Hydrogen, with higher heat
transfer coefficient, would be even preferable except that safety issues make
the transition difficult.
The advantages are environmental friendliness and reduced distortion, added to
the vacuum furnace processing that protects the surface from contamination,
saving expensive cleaning and finishing operations.
9.6 - Materials Characterization describes those properties and features
of composition and structure (including defects) of a material, that are
significant for a particular preparation or application of the material.
The goal is achieved by applying modern analytical methods while understanding
the most common applications and limitations of each method. This discipline is
evolving rapidly as new material requirements and testing methods are developed.
Materials Characterization development of technologies and tools is especially
important in the new and expanding field of nanotechnology and more generally
for any new material with advanced properties.
The list of the common analytical and testing methods available to the engineer
is too long and complex to be of use to the average technologist. Specialized
advice is to be sought for, by explaining to the person familiar with the
methods and techniques the particular aspect of the problem confronted and the
nature of the answers required to solve it.
A recommended reference book on the subject is:
10 - Explorations: beyond the Welder
The Winning Essay on Relativity Einstein on the Photoelectric Effect Science Books for the Young Beyond Einstein On Popular Science
11 - Contributions: Your Experience
There is not much to relate in this Department, possibly because of the vacation
season when everybody that only can do that, leaves his/her desk and climbs the
mountains or plunges into the sea.
Two different readers facing repair jobs with cast iron asked for
recommendations on the use of proprietary SMAW welding rods, whose manufacturers
claim they permit welding without preheating. Unfortunately we had no experience
with those names.
But now at least those two readers know if the claims of the manufacturers were
real and believable or not. Why would not you share your experience with your
fellow readers?
We would like to include in this publication short descriptions of successes
with your house jobs, using whatever products and techniques that actually
helped you.
We invite anybody having useful and practical information to write us by e-mail
for inclusion in our next issue. Click here. Our readers with
similar problems will be interested and might contact you for details. A perfect
occasion for making new friends.
12 - Testimonials
From: "isik celik ltd" 'isikcelik@efes.net.tr' Thank you for your kind interest -----------------------------
From: " [iso-8859-1] pankaj wani" 'pankajpankaj2k21@yahoo.co.in' Respected sir,
13 - Correspondence: a few Comments
Some of the correspondents, possibly time pressed, write just what they want,
without a word of explanation or details on the job. Some of them seem to
forget that there is no reason for them to be impolite.
It should be clear that people who do not provide required information to
explain their question possibly do not know what they really need: it is
therefore quite impossible to help them. They should welcome the
opportunity of discussing their application, which is the best way to obtain
meaningful information.
But if you ask only for the short, complete data sheet with all the answers,
more often than not you only give evidence that you do not realize that on your
subject huge budgets were expended, years of trials and testing were employed,
tons of articles and volumes of handbooks were written.
So, please treat yourself and your question with all due respect, and be aware
of the fact that the details you should provide are only needed to give you a
complete and useful answer also on those aspects you might have overlooked.
14 - Bulletin Board
14.1 - I fully enjoyed my short vacation announced last month, thank you.
I hope that the delay in providing the required answers did not cause undue
inconvenience.
14.2 - A Conference and Show called Furnaces North America has been announced
that will be held at the Nashville (Tenn.) Convention Center and Renaissance
Nashville Hotel, September 23-24, 2004. Produced and sponsored by Metal Treating
Institute (MTI), (www.metaltreat.com), Jacksonville Beach, Fla. For information
visit www.fna2004.com
14.3 - Our Site Host, SiteSell.com, has finally released their complete solution
for Store handling on the Internet, for small and home e-commerce business.
As usual their offer is self contained and needs not external support. For
curious and enterprising people it makes sense to explore what benefits are
proposed now. We recommend them because we know that they are serious and
helpful.
Start with free Downloads: Good luck. See you next time.
Copyright (c) 2004, by Elia E. Levi and welding-advisers.com, all rights
reserved
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