PWL#076 - GMAW-P, Straight CO₂ gas GMAW, Metal Cored Filler Wire, Delayed Failure, Electrochemical Machining, Hydrogen Embrittlement, Interview with Marcel Bauer, and more...

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2 - Article - Pulsed GMAW (GMAW-P)

3 - How to do it well: GMAW with straight CO₂ gas.

4 - Filler Metals Press Release

5 - Online Press: recent Welding related Articles

6 - Terms and Definitions Reminder

7 - Article - Delayed Failure

8 - Site Updating: Electrochemical Machining, Hydrogen Embrittlement

9 - Short Items

10 - Explorations: beyond the Welder

11 - Contributions: Interview with Marcel Bauer

12 - Testimonials

13 - Correspondence: a few Comments

14 - Bulletin Board

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1 - Introduction

The last issue of this year, No. 76 of Practical Welding Letter, opens with a few answers to questions asked by a reader. I am grateful to my readers for the opportunity to deal with what interests them, and I invite everyone to let me know the hot problems they have.

In fact Pulsed GMAW is not needed for every job, but there are certain projects where its skillful application is definitely cost effective providing better quality welds with higher productivity.

Answering to another reader's question, I briefly comment on the suitability of straight carbon dioxide to serve as shielding atmosphere for GMA Welding. The final decision depends much on availability and cost, but for inquisitive readers a reference is given to an authoritative source that can dispel some doubts.

In the Filler Metals section a press release is proposed. Although it is about a commercial product, the information presented can be of help to anybody looking for improving specific weld properties in demanding applications.

The article following was written in answer to an intriguing case history described by a reader. Although it cannot substitute for a full Failure Analysis that should be based on the complete knowledge of the case and on investigation of the actual failure using all needed metallographic tools, nevertheless the answer gives an indication of the most probable cause for the accident reported.

In the Site Updating section two new pages are presented. One refers to some characteristics of the Electrochemical Machining process, which permits to shape intricate geometries in hard materials without introducing surface stresses. The other reviews the practical aspects of preventive measures that should be applied to avoid the risks of Hydrogen Embrittlement.

A new interview is published with a reader that kindly shares his experience and opinions summing up a productive career in welding. Readers who promised to send in their contributions are reminded to kindly keep their word.

Other sections are where they should be expected. Your feedback is always welcome. Please don't use REPLY.
Use the Contact Us form instead.

2 - Article - Pulsed GMAW (GMAW-P).

A reader asked the following

Questions:

1. What are the essential welding parameters in Pulsed GMAW process?
2. How does Peak current and Base current affect the properties of weldments?
3. How to calculate Heat Input with this method?

Answers:

1) In GMAW-P, the welding current is pulsed alternatively and periodically between background (or base) and peak (or pulse) levels. Each cycle is called a period, and the period is repeated several hundred times per second.

The main setting parameters which influence weld quality and wire melting are Background Current I_b, Peak Current I_p, Background Time T_b, Peak Time or Peak Duration T_p and wire feed rate for any given wire size.

Optimal values of these parameters should be set to ensure drop transfer mode, the so called "one drop per pulse" mode (ODPP) in order to obtain proper weld quality.

The benefit of the pulsed energy is that it produces desirable fusion characteristics, while reducing considerably the heat input when compared to axial spray transfer or other welding processes.

When operated within carefully controlled parameter limits, this process variation from standard GMAW has wide application because of its many advantages, including the following:

• Reduced spatter level, compared to short-circuit and globular transfer, and minimal postweld cleanup,
• Increased deposition rate and higher travel speed,
• Higher metal transfer efficiency (98%),
• Lower fume levels, and a healthier environment for the welder,
• Lower hydrogen weld deposit and improved Charpy Impact test values,
• Lower heat input, resulting in less distortion and improved weld quality,
• Excellent weld fusion and X-ray quality weld deposits with better penetration control,
• High flexibility, suitable for all welding positions on thick or thin base metals,
• Usable in place of short-circuit transfer (GMAW-S) with lower risk of incomplete fusion.

2) The current levels are only two of the parameters determining weld quality. To understand parameters function, the period can be broken down into several parts:

1. Ramp up rate increase stiffens the arc, reducing the arc column size and increasing the propelling force throwing molten drops detached from the electrode,
2. Overshoot, an instantaneous current increase above the peak current level, necessary to overcome the influence of inductance, variable with cable length,
3. Peak current level directly controls penetration: the higher the peak current, the deeper the penetration,
4. Peak Time or Peak Duration influences the width of the bead shape,
5. Tail-out drop time, speed of change and shape controls energy addition to the molten droplet, directly influencing puddle fluidity and penetration,
6. Sudden current drop down to background level, reduces the tendency for fine droplet spatter,
7. Background Current, the low-level current needed to maintain the arc,
8. Frequency or Repetition Rate shapes the weld bead by narrowing the arc cone with increased frequency.

3) GMAW-P Heat Input calculation is the subject of disagreement among researchers. According to the conclusions of a review paper:
"The most appropriate method to calculate heat input for GMAW-P is through Average Instantaneous Power (AIP), which requires the use of high speed data acquisition."

See:
Assessing the Effects of GMAW-P Parameters on Arc Power and Weld Heat Input
http://files.aws.org/conferences/abstracts/2003/01b.pdf

3 - How to do it well: GMAW with straight CO₂ gas.

Q: I see a lot of GMAW being used around the place but they are using a straight CO₂ gas. In my experience, we always used a CO₂/argon mix gas which not only produced a better weld, had less spatter also.

There also was some discussions with my American colleague who mentioned in the States they will not allow the use of GMAW with only CO₂ (carbon dioxide) Gas. Apparently there were some failures resulting from this process.

Is there anything to back this theory up and have you heard anything similar?

A: The use of CO₂ is not forbidden as far as I know, so that failures, if there were any, may be due to other causes too. Gas selection is probably also a question of cost and of ease of supply, especially in developing countries.

You are right about spatter and quality, but if Welding Procedure Specifications (WPS) were approved according to requirements of applicable Codes there is nothing wrong in using straight CO₂.

A thorough presentation and discussion of Shielding Gases for GMAW can be found at page 64 of ASM Handbook Volume 6 that is a fundamental reference book for anyone involved in welding. If you want to purchase your personal copy you can order it online at
http://www.techstreet.com/cgi-bin/joint.cgi/374542/cgi-bin/detail?product_id=246403

4 - Filler Metals Press Release

Hobart Brothers' Metal-Cored Wire Offers High Impacts at Sub-Zero Temperatures

TROY, Ohio. October 28, 2009

For welding on nickel-molybdenum steel applications that require high impact toughness at sub-zero temperatures, Hobart Brothers offers its Tri-Mark® Metalloy® 80N1 metal-cored welding wire. This gas-shielded wire has been specifically designed for both single- and multi-pass welding on 1/2 Ni-1/4 Mo, 1 Ni-1/4 MO and 1-1/2 Ni-1/2 Mo steels, such as those used for castings or heavy equipment. The Metalloy 80N1 wire can also be used for welding on weathering steel applications where color match is not critical. It meets the E80C-Ni1 classification per AWS A5.28 specifications.

Useable with either 75% Argon/25% CO₂ or 98% Argon/2 % O₂ shielding gas mixtures, Metalloy 80N1 wire provides tensile strengths ranging from 85,900 to 90,000 psi, respectively, and maintains its toughness at temperatures as low as -50 Fahrenheit (-46 Celsius). The wire also offers similar benefits as Hobart Brothers other premier Tri-Mark metal-cored wires. These include: high deposition rates, low spatter levels, excellent penetration and wetting action, and faster travel speeds (as compared to solid welding wires). Combined, these features help improve productivity and reduce costly and time-consuming rework.

Metalloy 80N1 can be used in the flat, horizontal and vertical down welding position using the CV (constant voltage) spray mode or in all positions using a power source with pulsing or short arc capabilities. It is available in .045-, .052- and 1/16-inch diameters in a variety of spool and drum packages and sizes.

5 - Online Press: recent Welding related Articles

Submerged Arc Welding Optimization
http://www.thefabricator.com/ArcWelding/ArcWelding_Article.cfm?ID=2345

The Advantages Of New Fibre Laser Welding Techniques
http://www.officialsitebuilder.com/blog/1290/the-advantages-of-new-fibre-laser-welding-techniques/

Basic Tips to Improve Plasma Cutting Performance
http://www.millerwelds.com/resources/articles/Plasma-cutting-for-home-hobby-motorsports-art-and-DIY-projects-techniques-results/

How To Successfully Weld Aluminum with a Compact MIG Welder
http://www.lincolnelectric.com/knowledge/articles/content/compactmig.asp

TWI Connect - November December 2009 (8 pages)
http://www.twi.co.uk/content/connov09.pdf

6 - Terms and Definitions Reminder

Automatic Welding is performed with automatic equipment that requires no controls adjustment by an operator.

Braze is the joint resulting by heating an assembly to the brazing temperature using a filler metal whose liquidus is higher than 450 ⁰C (840 ⁰F) and lower than the solidus of the base metal.

Double Welded Joint is a fusion welded joint that is welded from both sides of its thickness.

Friction-surfacing, a modification of friction welding, is a solid state process whereby layers of various metals are deposited upon a metallic substrate.

Gas Cylinder is a portable container used to store and transport compressed gas.

Hot Pressure Welding is a solid state welding process that produces a weld by application of heat and pressure sufficient to cause macro deformation of the base metal.

Reduced Section Tension Test is performed on a specimen where, in the middle of its reduced section, is located a transverse section of the weld.

Thermite Welding is a process that realizes joining of two parts by fusing them together with heat and filler metal given off by superheated molten metal from exothermic reactions, poured in a mold assembled around the joint.

7 - Article - Delayed Failure

A reader sent us the following letter:

We have built a spreader bar out of QT 100 steel which has been designed by our customer. All the proper steps have been taken to prevent cold cracking, i.e...
pre heat with ceramic pads to 350 F, not allowing interpass to exceed 450 F, slow cooling by wrapping in insulation.

Twelve 30' sections have been built and 100% mag particles inspection was performed 24-72 hours after completion. The NDT was accepted and the parts were shipped.

One year later the bars were reinspected before putting them into service.
Several transverse cracks were discovered in the same location of every bar. The main plate is 1 1/4" thick and a 3/4" doubler plate is welded to it.

There is also a half pipe running down the center acting as a lateral stiffener.
There is a 1 1/8" space in which two multi pass fillets must be welded joining the doubler plate to the main plate and the half pipe next to it.
I believe there is great residual stress in this area due to the design of this bar and would like any opinions to help me.

Answer:

Too many essential details are missing from this exposition so that it is difficult to reach conclusions, but the reader may be right as to a concurrent cause of failure.

In principle one should:

1. Perform failure analysis on the several failed parts,
2. Plan how to repair the cracked parts,
3. Revise the procedure for future application.

Although a long time passed, one should verify if the sound parts are also in danger of cracking in service.

While the standard precautions were applied both before and during welding, it appears from the results, unfortunately, that they were probably inadequate.

Transverse cracks in high strength steel weld metal are generally the result of longitudinal shrinkage strains acting on weld metal of low ductility, typically related to hydrogen embrittlement.

Hydrogen induced material degradation is one of the well known causes of delayed fracture.
The visual examination of fractured surfaces might possibly confirm this hypothesis.

Hydrogen cracking is often referred to as cold or delayed cracking because of the tendency for these cracks to occur and propagate after the weld has cooled to ambient temperature.

It is not uncommon for such cracks to be revealed hours or even days after welding, depending on the restraint conditions within the structure.

For this reason it is common practice to postpone the final nondestructive testing until several days after completing a weld.

For a more detailed description of Hydrogen Embrittlement, readers can see the new page introduced in section 8 hereafter.

Check also the following Book:
Welding Steels Without Hydrogen Cracking, 2nd Edition
Number of Pages: 160
Price: $25.00 (ASM Member: $20.00) - Priced for clearance sale!
Hardcover 06324G

Go to
http://asmcommunity.asminternational.org/portal/site/www/
Click on
Materials Information
Click on
Books and CDs
Go to the bottom of the page, where it says:
"Discover more than 1700 titles from ASM and other leading publishers."
Type the above title in the Search Box and
press the Search button.
A list of 6 out of 642 results appears.
Click on the first which is what we look for.
(Wait patiently for a while)
Buy the book and enjoy it!

8 - Site Updating: Electrochemical Machining, Hydrogen Embrittlement.

The Pages of this Month deal with a useful process and with a dangerous hindrance.

Electrochemical Machining is a non contacting electrolytic method of metal removal used to work hard metals and difficult to machine alloys. It produces unstressed smooth surfaces ideally suited to severe operating conditions.

This process should be kept in mind when dealing with complex shapes or with hard to machine materials. Click on Electrochemical Machining to see the new page.

Hydrogen-Embrittlement can cause delayed cracking and fracturing after welding if occasional sources of hydrogen are not excluded from the weld region by suitable precautions. Stress relieving and baking should be implemented. Delayed inspection is recommended.

The risk is higher for higher strength steels and for service exposure to low temperatures.
Click on Hydrogen Embrittlement to reach this new page.

To stay updated on new additions and edition of pages to the website, readers can subscribe to the RSS feed as explained in every page of the website under the NavBar. The Site Map can be consulted for finding specific pages.

Comments and feedback are welcome. Don't use REPLY. Use the Contact Us form instead.

9 - Short Items

9.1 - Magnetic Particle Inspection is a nondestructive method of inspection for determining the presence, orientation and extent of surface and near surface cracks and similar imperfections in ferromagnetic materials. Contrast colored or fluorescent (under UV light) finely divided magnetic particles, applied dry or wet to the magnetized part, are attracted to and outline the pattern of any magnetic leakage fields created by discontinuities.

9.2 - Plane Stress is the stress condition in linear elastic fracture mechanics in which the stress in the thickness direction is zero; most nearly achieved in loading very thin sheet along a direction parallel to the surface of the sheet. Under plane-stress conditions, the plane of fracture instability is inclined 45° to the axis of the principal tensile stress.

9.3 - Ultrasonic Inspection is a nondestructive method in which beams of high frequency sound waves are introduced into materials for the detection of surface and subsurface flaws in the material. The sound waves travel through the material with some loss of energy (attenuation) and are reflected at interfaces. In one method, the reflected beam trace is displayed and then analyzed to define the presence and location of flaws or discontinuities. Most ultrasonic inspection is done at frequencies between 0.1 and 25 MHz.

9.4 - Vapor Degreasing is performed in the vapor, considerably heavier than air, over a boiling liquid solvent. Vapor condensing on the cool surface of workpieces drips back removing oily soil soluble in the solvent.

9.5 - Weathering Steels are copper bearing high-strength low-alloy steels that exhibit high resistance to atmospheric corrosion in the unpainted condition.

9.6 - X-Ray Diffraction (XRD) is an analytical technique in which measurements are made of the angles at which x-rays are preferentially scattered from a sample (as well as of the intensities scattered at various angles) in order to deduce information on the crystalline nature of the sample, its crystal structure, orientations, stresses and so on.

10 - Explorations: beyond the Welder

The Discovery of the Lost City Hydrothermal Field, 2000
http://www.lostcity.washington.edu/science/discovery.html

Financing Solar Energy Systems (44 pages)
http://www.nrel.gov/docs/fy99osti/26242.pdf

Plug in America
http://www.pluginamerica.org/

NASA's WISE Mission Arrives at Launch Site
http://www.jpl.nasa.gov/news/news.cfm?release=2009-127

Bridge in a Backpack Construction (video)
http://ceramics.org/ceramictechtoday/materials-innovations/video-of-the-week-bridge-in-a-backpack-construction/

11 - Contributions: Interview with Marcel Bauer

Q: What did appeal to you when you first considered welding as your Career?
A: Decent wages, interesting work, and my Grandfather was a welder.

Q: How did you start your welding Career?
A: Got a job where my Grandfather worked.

Q: Did you plan to achieve definite Career milestones in a given timeframe?
A: No.

Q: In which field of welding are you active?
A: GTAW and general fabrication.

Q: Which were your major achievements during your Career in welding?
A: Learning to do very good quality work, always.

Q: Which of your achievements procured to you the highest satisfaction?
A: Doing excellent work, first time, every time.

Q: Which challenges did you find hard to overcome?
A: Welding aluminum that had been in salt water for a very long time.

Q: What did help you in persisting and overcoming the difficulties?
A: Removal of all salt from the metal pores.

Q: Which is the most important lesson you would like to transmit to young welders?
A: SAFETY FIRST! Protect your skin, your eyes and your lungs. Always wear your safety equipment!

Q: Which goals should a young welder aim to reach?
A: Top quality work, always be a professional in work and attitude.

Q: Do you foresee interesting developments likely to change the welding profession?
A: Better equipment allows easier and more precise quality work.

Q: How would you invite young people to consider welding as their Career?
A: There is decent money to be made and great job satisfaction. Remember, there will always be a need for skilled welders/trades people.

Thanks to Marcel Bauer for having shared with us his experiences. We look forward to be able to publish other interviews from readers who accepted to send us their interesting stories.

12 - Testimonials

From: Mahsa Seyyedian
To: Welding Advisers
Date: 07 Nov 2009, 05:16:05 AM
Dear Sir,
I am one of your members receiving Practical Welding Letters.
Thank you so much for the good information you give us.
[...] Regards,
Mahsa Seyyedian

From: Hassan Kheir To: Welding Advisers
Date: 05 Nov 2009, 01:54:24 PM Subject: Re: PWL#075 - Sinter Brazing and Resonant Acoustic Inspection, Brazing Flux Removal, Active Solder

Hi Levi,
Thank you for this valuable information...
This is exactly what I am looking for...
Please keep me in your list and forward what ever you have about Welding and metals...
Best regards.
Hassan Kheir

13 - Correspondence: a few Comments

13.1 - I generally believe that readers asking questions are genuinely interested in the subject either for intellectual curiosity or for practical reasons. Although it would help to have an idea of the purpose of the question in order to keep the answers useful, quite seldom the reason for asking is exposed.

What I cannot understand and what bothers me is the attitude of some readers who keep shooting to me unrelated questions on the most diverse welding arguments without explaining or justifying the reason for their interest.

In most cases such a behavior is interpreted as a useless pastime. I would like to request readers to ask only what they need to know for definite purposes. Thanks.

13.2 - Sometimes readers would like a short answer to questions that need textbooks to be answered fully or that are related to controversial explanations not yet decided. Although readers may not be aware of the weight of their query, they should understand that a short letter cannot provide the useful insight sought.

13.3 - While scores of readers successfully downloaded my Hardness Book, a few still write me they could not download. Either they did not read attentively the page giving instructions, or they missed the validity time of the password (changed once a month).

14 - Bulletin Board

14.1 - PICALO 2010, Pacific Int'l Conf. on application of Lasers and Optics
March 23-25, 2010, Shangri-La Hotel, Wuhan, P.R. China
www.laserinstitute.org

14.2 - WESTEC 2010
March 23-25, 2010, Los Angeles Convention Center, Los Angeles, Calif. USA
www.westeconline.com

14.3 - The Japan Int'l Welding Show 2010.
April 21-24, 2010, Tokyo Big Sight, Tokyo, Japan.
Organized by The Japan Welding Engineering Society and Sampo Publications.
www.weldingshow.jp/english

14.4 - SiteSell Services Case Studies
These case studies, from users from around the world, show how it is possible to take advantage of the uniquely qualified SiteSell Services Specialists to build and grow your online business.
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