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PWL#004 - Submerged Arc Welder, F/M for Alum. Alloys, Fatigue Failures, sorting Stainless Steel November 30, 2003 |
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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 -----------------------------------------------------------------------
You are urged to pass-along this publication to your ----------------------------------------------------------------------- Date: December 2003 - Practical Welding Letter - Issue No. 04
------------------------TABLE of CONTENTS---------------------------
==> 1 - Introduction ==> 2 - Article: Your Submerged Arc Welder ==> 3 - How to do it well: Sorting unweldable stainless ==> 4 - Filler Metal: Selection of Aluminum Alloys ==> 5 - In the press: Recent Welding and related Articles ==> 6 - Terms and Definitions Reminder ==> 7 - Article: Fatigue failures ==> 8 - Site Updating ==> 9 - Readers' Contributions and Short Items ==>10 - Explorations: beyond the Welder ==>11 - Correspondence: a few Comments ==>12 - Bulletin Board ==>Unsubscribe link
1 - Introduction This new issue of Practical Welding Letter is the first one where we could enjoy the help of some targeted feedback, pointing us in the direction of certain items which seem to be requested by at least a certain number of readers. We hope that the trend of providing us with the readers' wishes will continue, to improve the relevance of the content. After the presentation of the Feedback Form (sorry for the technical problem of the first trial) we were delighted to see a substantial stream of input from our readers. We are encouraged by the positive feelings expressed in many of your messages and we certainly wish to provide significant information. As expected we have now more subjects on hand than can be dealt with in a single issue but, if not now, we will do our best to answer in one of our next publications. Although not as diffused as other forms of manual joining, the Submerged Arc Welding process seems to gather quite a bit of interest: therefore we undertake to handle it in some depth. Aluminum alloys are in demand as a subject to learn more on, and filler metal selection is possibly not obvious: that is why a section is dedicated hereafter to supplying at least some information on the criteria to keep in mind when selecting the filler metal most suitable for the job at hand. Some articles referred to in the review of the Press deal with problems of Welding Education and Training. It appears to be a hot subject, because apparently there is a perceived and persistent lack of experienced welding professionals. It was therefore justified to dedicate one page of the Site to this problem, as indeed was done. Now more than before we urge our readers to take a few minutes
of their precious time and to let us know their comments and
wishes. Use the prepared form: click here. 2 - Article: How to select Your Submerged Arc Welder Note: This subject came up as requested by some of our readers, who were so kind to provide us with their feedback, as urged by us to do so in the Form we provided in the Subscription page of our Site. We are glad to know what interests you more, and we will deal in next issues with other demanded subjects. Submerged Arc Welding (SAW) is best suited to provide high productivity solutions to manufacturing problems arising when welding relatively thick material (over 6.3 mm = 1/4") in flat or horizontal position with long joints making the most of mechanized set-ups. Multiple passes are applied when a single pass would not provide an acceptable solution, either economically or metallurgically. Although semi automatic manual application is also possible, it is much less attractive than the above. The process consists in establishing in the joint an electric arc between a running electrode (driven by power rolls through contact tips), in the form of a solid or a cored wire, and the workpiece, under the cover of a granulated flux. Mechanical means provide the relative motion of the joint relative to the electrode holder. Advantages: High deposition rate and speed of welding. Use of not so skilled workforce. Suitable to operate even in presence of moderate air drafts. No need for protecting gases. No visible arc and no fumes, friendly to the environment. High quality generally available. Suitable for depositing hardfacing or corrosion protective layers. Suitable for adjusting the chemistry of the deposit by proper composition of the flux. Suitable to the manufacture of heavy wall tubes. Limitations: Needs precise set up and fixtures. Needs manual or mechanical removal of fused slag. Usually not suited for thin sheets (less than 3 mm ~1/8"). Only for flat or horizontal position. The most important part of the equipment is the power supply, which must be rated to the provision of elevated current for as long as needed. That means Duty Cycle = 100%, that is nonstop operation. See the Amperage Rating section in the following. Direct Current Direct Constant Current from a power supply exhibits a drooping characteristic curve expressing the dynamic relationship between current and voltage. The System has usually a voltage sensitive control device that varies the electrode feed rate in response to a change in arc length: feed is increased (to reduce arc length and voltage) when voltage goes up, or reduced (to increase arc length and voltage) when voltage decreases. As a consequence of the electrode variable feed rate the voltage remains quite constant at the predermined value, while the power supply provides constant current during the welding process. It is reported that this type is more difficult to set up because any change introduced either in the wire feed control or in the current has an influence on the other parameter. This type of power supply is preferred for thick joints using relatively thick filler wire. Direct Constant Voltage (or Constant potential) power supplies are self regulating. The voltage is set in advance at the power supply. The electrode is provided at a constant, predetermined feed rate. Current is determined by electrode size and speed: to increase the current one has to increase the wire feed and vice versa. So it is easier to set up. This type of power supply is preferred for relatively thin materials to be welded at high speeds. Polarity With both types of direct current as introduced above one should be aware of the consequences of the polarity selected. Selection affects the shape of the bead, the dilution of the base metal and the deposition rate. The least dilution and penetration is obtained with direct current straight polarity (electrode negative). This should be the choice for overlaying or hardfacing, and to reduce cracking tendency. For overlaying, the filler material can be in strip form, requiring proper rollers in the feeder. The plates should be clean, to limit porosity. Conversely direct current reverse polarity (electrode positive) provides increased penetration with thinner beads, permitting decreasing grove width: this in turn permits the use of less filler (increased productivity) and lower heat input (less distortion). It will also produce less porosity even with rusty or not so clean plates. Alternating Current The shape of the bead will result somewhere in between the shapes of both polarities discussed above for direct current. Alternating Current power supplies are used for SAW, with provisions for a square voltage wave shape. This modified wave form has a very rapid passage through zero voltage, which helps avoiding arc black out. Alternating current is beneficial at elevated current values, where direct current may be subject to magnetic blow interfering with welding. For extremely high weld deposition rates, special equipment with double or multiple electrode may be employed but we are not going to deal with that here. Materials The process is used mostly for welding low carbon and low alloy steels. Filler metal is covered by AWS A5.17. If special requirements are not present (like minimum assured impact strength or fracture toughness), it can be used also for less weldable steels although with proper provisions, like special consumables, preheating etc. Stainless steel can be welded by this process. Cast iron could be welded with nickel electrodes, although the application may not be simple. Aluminum, magnesium, titanium, lead and zinc are not suitable for SAW. Amperage rating The amperage range of any available power supply determine in large measure the thickness welding capability of the equipment. As a guideline light gages (6 mm ~ 0.25") can be welded by 300 Amp. while heavy plates (25 mm ~ 1") may need 1000 Amp. or more. The duty cycle determines the maximum percentage of welding time available in any ten minutes interval. A power supply with high maximum amperage but limited duty cycle, guarantees a higher duty cycle at the lower current level used in practice. However it is quite unusual that a thick weld should be welded in a single pass: this is because too much heat would be input, too much filler would be used and much distortion would occur. And the welding speed would result unacceptably low. Therefore it is customary to report in tables the current range for different filler wire sizes. It should be noted that different sources provide quite different parameters. The ranges displayed are large and show some overlapping. These are estimated values that are influenced also by flux, wire type, stickout, joint type, metal thickness and power supply. At any given current value, changing to a smaller size increases the current density and provides for deeper penetration and thinner bead, while the arc results more stable and the deposition rate increases. In the following Table I some initial parameters were listed for helping in setting up a Submerged Arc Welding operation for steel sheet and plate.
Consumables The composition of different fluxes is finely tuned to control the composition and the mechanical properties resulting in the weld, so that their selection should be based on careful consideration of the base metal and of the requirements. It is the combination of the filler metal and of the flux that provides the required properties. It is recommended therefore to seek advice from the manufacturers for chosing suitable and compatible consumables. Fluxes of two types are supplied: Fused fluxes are melted after mixing the ingredients, and then crushed, screened and packaged. They do not contain alloying elements or active deoxidizers. Bonded fluxes can be blended more easily with useful additives. They are however more susceptible to contamination and moisture. Filler metals for steels are listed in the following: "Specification for Low Alloy Steel Electrodes and Fluxes for Submerged Arc Welding"
Relative motion The accessories for providing relative movement as required
depend on the application and are independent on the power
supply selected, although the weld speed should be controllable
as a function of other welding parameters. 3 - How to do it well: Sorting unweldable stainless Q: A batch of regular stainless steel fasteners to be welded on a sheet, was inadvertently mixed up with free-cutting stainless ones. As we are told that free-cutting stainless is not weldable, how can we sort out the weldable items? A: Hopefully the batches, although unknown, are still separated: if this is the case you need only to examine representative specimens of batch A vs. batch B. It is quite straightforward to sort the types by microscopic examination, after grinding and polishing one surface of each one (a nondestructive process): the free-cutting material is peppered by sulfur particles readily standing out of the background. Alternatively you may try to weld: the nonweldable will crack right away... ----------------------------------------------------------------------- 4 - Selection of Aluminum Alloy Filler Metal Gas shielded arc welding processes, which can be performed without the use of flux, are the most employed for joining Aluminum Alloys. These processes permit full view of the welding being performed, allowing skilled welders to obtain quality work without costly flux removal and neutralization treatments. This article is intended to provide information and guidelines for selecting the most appropriate Aluminum Alloy Filler Wire for any job, by highligting the principal concerns to be considered. The accepted document governing the subject is: Additional relevant data can be found in: The following Table I lists the most important Aluminum Alloys Filler metal composition and melting range. The prefix ER indicating Electrodes and Rods is omitted from the list of items. The designations are followed by a number [x] which is used in Table II further down, for indicating the suggested selection according to specific criteria.
Notes: The primary factors commonly considered when selecting a welding filler alloy are:
A further consideration, not listed here, but sometimes important refers to the behavior of the weld to somewhat elevated temperatures, or to applications in very cold environments within the limits suitable to aluminum alloys. In the following Table II the above designations, indicated as [x], are listed as suggested choices for satisfying the criteria of each of the headings. The indications are not absolute and should always be tested to confirm their suitability to the requirements of any particular application.
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Notes: In the following Table V are listed the suggested initial parameters that can be employed for welding Aluminum Alloys with the Gas Metal Arc Welding with Filler Metal Electrode Wire.
Notes:
5 - In the Press: Recent Welding and Related Articles
For those taking welding for granted, a sobering reminder
from The Welding Institute, UK, is available in an article,
actually a report of a Seminar,
on Weld Failures and how to avoid them.
Actually it is a call for managers and for everyone involved,
on the Importance of Welding Education and Training.
Note: The link that we provided was live in the past. TWI removed the page or filed it somewhere else where we are unable to find it. Sorry!
From the Canadian Welding Association an article relates on
programs and initiatives to spread Welding Education in Canada.
A strong drive from industry is essential.
The article published
in the Spring 2003 issue of Canadian Welding Association Journal
is now (03/2006) available to members only. See From AWS Welding Journal an article titled "Spot welding: still in
the Driver's Seat" explains that robotics, repeatability and economy drive the quest for new and improved machines.
For gaining a comprehensive view on the Aluminum Industry in the U.S. one can
click here. -----------------------------------------------------------------------
6 - Terms and Definitions
Reminder
Do you remember what is...
Autogenous weld:
A weld made by fusing the base metal without the addition of filler metal.
Burn Back:
A condition where the electrode wire is not properly fed to get the melting of the tip outside the torch, far from the contact tip (especially for thin, soft aluminum wire).
Magnetic Arc Blow:
A powerful deflection of a welding electric arc, due to the influence of a magnetic field present at the location interfering with the field generated by the welding current. Particularly troublesome with direct current.
Stickout Distance:
The distance between the last electrical contact to the electrode wire and the weld.
(In Metal Inert Gas, in Flux Cored and in Submerged Arc Welding).
Would you want other terms being presented in next issues?
Let us have your feedback on the Form. Click here. -----------------------------------------------------------------------
7 - Article: Fatigue failures
It is widely recognized that the majority of failures occurring in
mechanical elements of structures or machinery are due to the
fatigue phenomenon.
It is therefore quite important to understand their origins
and the prevention means applicable to the elements designed to
substitute the failed ones.
Fatigue is the progressive formation and propagation of cracks
in a material subjected to repeated or alternating stresses (due to
cyclic loads or forces), lower than its static yield strength limit.
The final rupture occurs when the remaining section is no longer
sufficient and able to sustain the normal operating forces.
Generally the appearance of the fracture presents a telltale aspect
which is easily recognized as due to fatigue, and possibly other clues
pointing to the origin of the
failure. Characteristic features,
commonly called "beach marks", represent progressive stages of crack
development, and are the single most important distinctive sign of fatigue.
One of the first tasks of the investigation is then to determine,
from the appearance of the fractured surface, assuming that it has not
been obliterated by contact wear successive to the fracture, the
mechanism generating the failure.
If it can be concluded with a fairdegree of certainty that fatigue
was involved, then the most common course of action would be to plan
the substitution of the element with a new one designed to better
perform the task of that which ruptured, trying to improve its
performance by taking into account the operating factors.
The contribution of an expert Metallurgist assisted by the equipment
and know how of a modern Laboratory can be invaluable, especially if
the case is complicate or tyed to legal responsibilities, or if
environmental or operational conditions like humidity, heat,
vibrations, very high speeds, impact loading, or corrosive agents
are involved.
However, for the simpler cases (i. e. a broken shaft), there are a number of less expensive solutions which may be preferred by the one who has to pay for.
The properties to be looked for are hardenability, that is the
capability to harden through the depth of the part, and hardness
after heat treatment, taken as an indication of strength.
Where can inexpensive help be found?
At those providers whose metallurgical expertise is included in the product or service they market and sell.
Service Centers representing reputable materials manufacturers
are not only stock keepers of metals: the best of them know their products in depth and are familiar with applications similar to the one for which a new piece of steel (most often) has to be selected from the overwhelming assortment.
They
may have informative leaflets displaying composition and
properties, usage and treatments, availability and costs.
They are worth listening to, they know what they say.
Even before cutting the first chip, the Heat Treatment job shop that will be charged with the processing of the part should be involved in the project and asked for advice.
He/she could be requested to test the hardness of the broken part, to obtain an indication of baseline properties which should be improved upon. At the same time one should inquire if some surface hardening might be present.
A proper metallurgical investigation would try to pinpoint the
cause of failure (faulty design, defective material, improper
treatment, surface damage or pure high cycle fatigue).
By skipping the quest for the cause, one implicitly assumes that
the failure occurred by chance as a consequence of one or more
contributing factors, that will not necessarily duplicate if the
marginal conditions are improved.
The heat treater will not only suggest the best treatment to
perform (most often hardening and tempering) and by what equipment, but will advise at which stage in the machining sequence of operations the processing should take place, will provide useful tips for control deformations and may offer to take care of Hardness Testing for control, and of Magnetic Particles Inspection for assuring the soundness of the part.
In addition he/she may be familiar with finishing processes,
like burnishing, shot peening or phosphatizing which will add value to and improve the properties of the final product.
Shot peening in particular is a process designed to induce on
the surface of the treated parts compressive residual stresses
that may effectively prevent the occurrence of fatigue failures.
A sketch of the part and the taking of dimensions will probably
be done in house with great care: a few remarks are in order
here.
Depending upon the exact position of the failure, one should
consider that sharp fillets or notches, known in general as stress raisers, may have a nefarious influence on fatigue life: they should be avoided or reduced in intensity (larger radius etc.).
Normally the smoother the surface finish the better, except that
grinding is known to introduce tensile stresses which are harmful
and even tiny cracks which are ruinous; grinding control, if
applicable, should be performed by a certain procedure known as
"Nital Etch".
The Heat Treater may know where to do that, especially if he is
involved with treating gears.
Summing up, although fatigue failure may have heavy and costly
consequences on maintenance and availability of equipment,
it can be controlled for the future if the new design and possibly improved procedures take care of most of the influencing factors which have been outlined above.
It should be added that, if
the breakdown of important pieces of
equipment could have harmful economic consequences, not to say
personal danger to people, one should always search for cracks
especially in rotating parts, during sessions of planned maintenance.
This is the only way to prevent fatigue failures, by retiring the
damaged part while still functioning but before final rupture.
Special crack detection techniques are available for performing
this non destructive inspection.
Is this subject important to you? Let us know your comments and
wishes. Use the prepared form: click here.
8 - Site Update
We are proud to inform our readers that we released and posted
in the Site a new page on Welding Education and Training.
To reach it click here. We also added two new pages on the Site on how the Welding Advisers Site was conceived and built.
One page deals with the lessons learnt from a book that opened unseen horizons to us, teaching what is important and how one must prepare to achieve results.
To see this page click here.
The other page explains how the amazingly simple tools readily available to everyone are used in practice to realize one's project of building a new site.
To go through this page click here.
Of course the original page announced in the last issue, detailing the story of why and how the Welding Advisers Site was conceived and built in a do-it-yourself simple and powerful way, is still there for our readers to see. Clickhere.
----------------------------------------------------------------------- 9 - Readers' Contributions and Short Items
In the previous issue I proposed the idea, for those of you
who create welded Artwork, to create a forum for exchange
of tips and ideas, and to show off your creations.
One reader expressed interest, but not enough to send
in his Contribution. I repeat the invitation to the larger
readership.
Maybe some of you have special experiences interesting and
useful to fellow welders: you are
urged to share them with
all of us, as well as comments on what you find in PWL and
what you would like to read.
Continuing as in the last issue, for lack of original readers'
contributions, I will briefly deal with different subjects
that were raised by readers as items of interest.
Welding of ASME SA335 P22 material, (2.25Cr 1Mo) creep
resistant steel. Welding of thick carbon manganese steel plate.
Why is there spatter in CO2 shielded arc welding? Hot wire GTAW process.
The hot wire process is used with the GTAW (TIG) or the PAW (Plasma) welding processes in applications where a high deposition rate of the filler wire is desired . Hot wire welding provides higher deposition similar to that of GMAW (MIG) but with better control of heat input, reduced dilution in overlay welds and deeper fusion in groove welds.
Welding together two dissimilar metals.
To weld successfully one must first know exactly which materials
are to be welded. Not all combinations are permitted or possible. With high alloy
and especially with high carbon, preheat is needed to limit or prevent the formation of hard (martensitic) structures that may induce cracking. When welding an austenitic stainless to a carbon steel, it is
recommended to weld a transition layer (buttering) of high nickel
on the carbon steel, and only after weld the part so prepared to
the stainless steel.
Passivation of stainless steel.
This finishing operation produces a (passive) protective layer of chromium oxide on the surface of stainless steel which provides an effective defense against corrosion attack from certain substances.
Before welding, any passive layer should be removed.
On austenitic stainless steel passivation is usually done by
immersion in an aqueous solution of nitric acid (mildly oxidizing) which has the additional advantage of eliminating all surface contamination traces of iron from previous manufacturing processes.
See complete information in ASTM
A967. Specification.
Welding magnesium castings and shapes.
Most magnesium alloys can be arc welded under protective gas,
but weldability is widely varying among different types.
In Magnesium-aluminum-zinc alloys high aluminum is helpful
while zinc is detrimental. The problem may be "hot shortness"
causing weld cracking. The selection of the most suitable
filler metal and of special conditions like preheating, if
necessary depends on the composition of the base metal and
on the amount of restraint of the joint.
Strength of welded aluminium joints.
This item is brefly treated in the article on Selection of
Aluminum Alloy Filler metal. In general the strength is
reduced locally in the Heat Affected Zone. If critical,
thorough consideration should be devoted to the effects
of welds on the strength of structures and assemblies.
----------------------------------------------------------------------- 10 -
Explorations: beyond the welder
Would you like to visit a big Library? Click here.
Or a big Museum? Click here.
Look at the birdie! Click here.
Maybe you would like to do something else? http://www.webexpeditions.net/
----------------------------------------------------------------------- 11 - Correspondence: a few Comments
Questions are welcomed and we try our best to answer as promptly
and as completely as we can. As already remarked, however, most
of time the questions are incomplete so that it is
difficult to figure out the real situation.
I would like to be able to convince the readers that essential
information consists in detailed description of base metal
designation, specification and
condition, filler metal
description, process applied and outcome obtained.
It would also help to understand the general situation, if it
is a manufacturing problem, a theoretical issue, a research
program or a school assignment.
And again, if the reader was helped by the answer I would
like to know: I can only hope that this work brings valuable
benefits to readers, so that they are encouraged to spread
the notion of the Welding Advisers Site to their friends and
colleagues, at home and at work.
----------------------------------------------------------------------- 12 - Bulletin Board
12.1 - We would not want to make this a permanent feature of PWL,
but we must admit to a mistake in which we incurred in our
last issue: in the Title the issue number was erroneously
written as No. 0030 instead of No. 003. We apologize for this.
12.2 - As our Hardness Book has by now reached quite a substantial
diffusion we
would like to have your comments as to whether it
contains useful information for you and if you would like it
expanded to include other subjects. On this and on any other one
let us have your valued feedback. Use the prepared form: click here.
12.3 - For a description of how a very informative book helped
us to prepare and publish this Site, click Make Your Site Sell.
12.4 - If you are curious as to how a seemingly complicate process
(practically building an Internet Site) can be mastered by using
simple tools, click on The Complete Site Build It! Reference Center.
12.5 - With many more requests of information than can be dealt
with in a single issue, it is inevitable that not all subjects
can be answered instantly. But
the more you send in feedback
the more your subject will stand out, especially if many readers
request its presentation and discussion.
So please send in your demands and they will be answered.
Click here. -----------------------------------------------------------------------
Visit the Welding Advisers Site at
Copyright © 2003, by Elia E.
Levi and welding-advisers.com, all rights reserved
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