Robotic GMAW Parameters, Cast Iron to Mild Steel Welding, Filler Metals for Magnesium Alloys, Joining Composites to Metals, Thermal Cutting Processes with secondary Water and more...

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Date: September 2005 - Practical Welding Letter - Issue No. 25

1 - Introduction

2 - Article: How to select GMAW Parameters for Robotic Welding

3 - How to do it well: Welding Cast Iron to Mild Steel

4 - Filler Metals for Magnesium Alloys

5 - Online Press: recent Welding related Articles

6 - Terms and Definitions Reminder

7 - Article: Joining Composites to Metal

8 - Site Updating

9 - Short Items

10 - Explorations: beyond the Welder

11 - Contribution:Thermal Cutting Processes with secondary Water

12 - Testimonials

13 - Correspondence: a few Comments

14 - Bulletin Board

1 - Introduction

With this issue we are entering in the third year of publication of our Practical Welding Letter and we will soon reach and pass the number of 2000 voluntary subscriptions. We hope our readers enjoy what they read and pass on to their friends and colleagues the information on this source of knowledge.

We present here our third article based on the work of Ed Craig that kindly gave us permission to quote from his publications. This is on GMAW for Robots, a painful subject because most of robotic installations seem largely under exploited for causes not readily investigated by most managements.

It is only by developing both process and robot expertise that one can hope to bring utilization of these costly items more in line with their potential capabilities.

We answer then to a recurring question on how to weld cast iron to mild steel. Nothing mysterious here, besides the application of knowledge on what happens when cast iron is heated to its melting range.

For the Filler Metals section we deal here with Magnesium Alloys. A limited number of materials are available. This is one more reason to select from them wisely.

Composite materials continue to be used more and more for load bearing structures. It is only natural that at some point they should be joined with metallic elements: some basic points are presented in the article that deals with this subject.

New experience is accumulating on the use of secondary Water as an auxiliary medium for Thermal Cutting Processes. In the contribution, we report to some extent on articles that appeared in the written press.

Other departments fall in their place with their usual information. We hope that most of our readers will find some useful and interesting facts and news. We would welcome your comments, feedback and contributions. To send us, click on Your Questions and Feedback.

2 - Article: How to select GMAW Parameters for Robotic Welding

This is the third article based on the original information provided in books, articles and video training aids by Ed Craig from www.weldreality.com
We are grateful that he permitted us to distribute from his knowledge and experience to our readers, who are referred to the original sources for more details. We will cover hereafter the most important lessons relative to setting up Gas Metal Arc Welding for automatic or robotic operations.

The following excerpts are from Chapter 11, from page 289 to page 434, from Ed Craig's book that can be purchased online from his site above. The title is: "A Management and Engineers Guide to MIG Welding
Quality-Costs-Training"

It is affirmed in the book that positive thinking and planning should be devoted to solve some of the most pressing problems of many robotic installations that suffer from under utilization due to lack of expertise, excessive down time, extensive rework and low production rates.

The inherent benefits of a robotic welding setup come to fruition if they are fully exploited, which unfortunately seems not to be the case for the majority of applications.

According to Ed Craig, only a change in management attitude, giving the proper priority to the requirement of weld process expertise for the personnel involved, will succeed in improving the situation. Furthermore it is necessary to master the manual GMAW process to correctly program the weld parameters for robotic applications.

The major benefit of a robot is the potential for increased productivity with consistent weld quality. Notes Ed Craig: "If you cannot control the process you cannot control production". A correctly programmed schedule will permit to reduce overwelding. Considering the weld deposition rate as the main parameter for judging productivity, robot production can be between twice and four times that obtained by manual welding.

Controlling the process is the only way that permits to control both Quality and Productivity. A correct balance must be struck between the need of welding fast and that of producing quality welds. Notes Ed Craig: "It is just as easy, and less costly, to produce a quality weld as it is to produce a poor weld".

But first reduce robot's down time. If tip replacement is the problem, then replacement should be done in time, taking care of the correct recessed contact tip position. For the elevated currents and wire speeds used by robots, recession should be 3/8 to 3/4" (10 to 19 mm) inside the nozzle. If this cannot be achieved with regular contacts, they may need to be shortened by cutting off the end (with a wire inside to maintain tip bore shape), and then dressed with a file.

Wire cast and helix (see explanations in section 6) of the purchased wires should be well within tolerance and give no problems with electrode stick out of up to 1.25" (32 mm). If however the tip of the wire drifts to the sides instead of running reasonably straight, proper attention should be given to the problem. A wire straightener may be needed.

Welding wire should be the object of strict receiving inspection, to consider the above and also surface finish and condition. Also gas manufacturers premises should be monitored twice a year to ascertain consistence of quality at the filling station.

Whenever designing a welding cell served by more than one robot, one should take into account that the simplest maintenance procedure (i.e. changing contact tip) performed on a robot, would shut down also all the other robots from the same cell.

Auxiliary pieces of equipment serving a robot, e.g. a wire feeder, should not limit the robot productivity. A wire feeder connected to a robot should be capable of running up to 1000 ipm (25 m per minute).

For weld start, voltage should be programmed some 2 to 5 Volts above that for normal run, to reduce weld start problems. The torch, suitable for heavy duty work, should be given special attention. Water cooled torches may not be completely reliable.

One should be fully aware of the fact that increasing wire stick out (electrode extension) permits increasing wire feed speed and weld deposition rate while keeping the current constant. To illustrate the influence of electrode extension (Wire Stickout) on other welding parameters, we refer in the following Table an example provided by Ed Craig.

Table
Influence of Wire Stickout:
GMA Welding of 2.4 mm (3/32") thick Steel
with 1.2 mm (.045") wire at 350 Amp.

Wire Stickout		Welding Feed Speed		Voltage	Deposition Rate
mm	in	m/min	ipm	V	kg/hr	lbs/hr
10	0.375	8.9	350	30	5.4	12
18	0.75	14	550	24	6.8	15
25	1.0	19	750	28	9.0	20
30	1.2	21	850	31	10.4	23

Notes:
First line - Manual welding
Three other lines - Robotic welding

It is important to note that the welding schedule written to program a robot should clearly state the wire feed speed and the wire stick out. Both data are needed for specifying the current for any program. If one were to specify only the current, this value could be achieved with a number of different parameters, and the schedule would remain undefined.

Understanding the wire stick out influence and the advantages of increasing it, provides practical means to increase travel speed and weld deposition rate without increasing the current value. This is particularly important for robot welding to exploit the potential of higher weld speed, vs. that achievable with manual welding.

In the development program Ed Craig recommends:

• to determine the maximum current applicable to weld the part without incurring in burn-through
• to vary the wire stickout to obtain the maximum wire feed speed
• to evaluate the resulting weld penetration profile by examining weld sections.

At the same time the wire size should be considered: when using very high currents, one should test if the next higher wire size (which is less costly on a weight base) at a slower wire feed (better for feeder and contacts), can give acceptable results.

The gas mix should be selected according to the principles exposed in the previous articles.

One of the problems that the inattentive management might meet when upgrading a manual welding production to robot welding is that, unknown and unrecorded, the manual welders had personal solutions to cope with lack of consistency and excessive gaps. Robots are not intended to cope with the unexpected.

Before making the transition to robotic operations, design engineers should be requested to tighten tolerances to practical limits, and only then the robot can be programmed for the most unfavorable situation. Designers should also avoid specifying too thin parts for robotic welding.

It should be normal practice for any company planning to purchase a robot from a manufacturer to negotiate and agree on the dimensional tolerances of parts and joints before specifying the robot performance.

It is important to leave problematic or difficult to reach welds to manual welders, keeping them out of the robot schedule. Fixtures should permit robot welding in the most favorable position. Clamps should not stand in the way of mig guns (obvious but not always verified).

For simple joints a mechanized solution may be preferable to a robot application.

Regular GMAW power supplies are recommended by Ed Craig in preference to pulsed mig with electronic waveform control, unless their necessity is demonstrated.

It should be noted that welding wire quality may vary widely among different suppliers as to arc stability and weld transfer characteristics. It is strongly recommended to look for good and consistent performance, not for the cheapest price per unit weight.

Three important Training Programs should be established before implementing any robot welding application.

1. Training for Weld Process Control Program developers
2. Training for Preventive Maintenance providers
3. Training for Robot Operator to provide weld process and robot programming expertise

The Weld Process Control Program has to be prepared for every robot weld job following the logical sequence detailed in the above book. The results must be checked by metallographic sections.

After completing the above program, the data common to any type of weld should be issued as company wide Welding Procedures. Then a Weld Qualification Procedure is to be set up for specific parts. This document would describe the results of all the tests performed once the relevant parameters were arrived at.

All the metallographic photos and all the tests performed on the parts shall be reported, and destructively tested prototypes shall be numbered and stored, for demonstrating to customers and third parties the thoroughness of the development program.

Then a detailed document, the Robot Operator Welding Procedure, has to be written for each welded part. A copy of it, enlarged to poster size, has to be made available to the operator in the robot cell. All the necessary welding and robot set up data are to be clearly indicated, including the reasons for the most common drifts and how to recover from them.

The best way to manage welding robots is to make one person, most skilled in programming and processes, responsible for the resulting productivity and quality. Including the authority to make equipment, process and people changes necessary to achieve maximum utilization of robots.

For more information readers are urged to seek the above mentioned book or directly from Ed Craig at www.weldreality.com

3 - How to do it well: Welding Cast Iron to Mild Steel

Q: - How should one weld Cast Iron to Mild Steel?

A: - Cast Iron is an alloy of iron, carbon and silicon. Other elements may be added for special purposes.

• Gray Cast Iron, is probably the most common type. With slow cooling, its excess carbon solidifies as flakes of graphite. Its chief advantages are easy machinability, good damping capacity (to absorb vibrations) and relatively low cost. It is divided in further classes according to typical mechanical properties. Some types highly alloyed and with improved mechanical properties can be considered unweldable.
• Ductile Iron, due to special additions to its chemical composition, presents graphite in spheroidal form and has the highest strength and ductility of unalloyed cast irons.
• Malleable Iron is obtained with a long and specific annealing treatment that transforms iron carbides from white cast iron into irregularly shaped graphite nodules.
• White iron, that is produced by rapid cooling from the solidification temperature, contains most of its iron carbides untransformed, is very hard and brittle and practically unweldable.

The mild steels to consider should have limited content of Sulfur and Phosphorus, which are known to contribute to hot shortness or the formation of cracks at the time of solidification.

Cleanliness and weld preparation are always most important. It is known that welding might produce brittle structures and in general reduce the mechanical properties of cast iron. However successful welds can be performed for useful purposes if one acknowledges the limitation introduced by the processes.

A successful welding process should not cause the formation of cracks during or after welding and should not introduce harmful or excessive residual stresses.

There is not a single welding process capable of welding successfully any conceivable combination of iron castings and steel. Furthermore one cannot point to a single filler metal rod or electrode to cover all possible cases. Therefore the problem is not simple.

Furthermore small iron castings behave differently from large and massive cast pieces. The mass has a great influence on the self quenching capacity of the parts and on the cooling rate after welding, directly affecting the obtained structures.

Any welding process produces two zones that undergo important structural transformations:

• The weld metal is that portion of base and filler material that were melted by the welding heat, were thoroughly mixed and then solidified quite rapidly. The resulting structure is mainly a function of composition. As the dilution of cast iron into the melt contributes a large proportion of carbon that is responsible for the hard and brittle phases resulting during solidification, due attention should always be employed to melt the minimum amount of cast iron.
• The heat affected zone although not melted, was heated to high temperature by the nearby weld heat. Most of its carbon, that was in form of graphite, went into solution in the phase called austenite. Upon rapid cooling this carbon enriched austenite transforms to the hard phase martensite, which is brittle and susceptible to cracking.

To control the properties of the heat affected zone, to reduce hardness and shrinkage stresses, one has to reduce the cooling rate. This is usually achieved by preheating the iron casting, either locally with a flame, if it is very large, or preferably in a furnace. And then, after welding, by letting it cool slowly, having wrapped or buried it into insulating material, or again in a furnace.

Preheat will also control the structure of the weld metal itself to the point that the formation of martensite is minimized or avoided at all.

However, small castings can be welded sometimes without preheat if the results are acceptable. Alternative techniques consist in welding thin and short beads. The heat from subsequent welding beads temper the hard structures generated by previous ones.

To reduce and redistribute residual stresses, peening of the still hot bead with a rounded ballpeen hammer should be performed. Further weld passes contribute to interpass heating that helps in preventing too rapid cooling.

In general one should try to heat to the least possible peak temperature, to introduce the minimum heat, using small electrodes and low currents, to apply suitable preheat, to control interpass temperature and to study the performance of different types of filler material until the satisfactory selection is found.

The consumables available for welding cast iron are quite varied. Filler Metals of different types can be used for welding cast iron to mild steel. The selection should be based on ease of performance and on the acceptable results achieved. Once the main factors are understood and taken care of, practice and trials can tell which is the most economic and best solution for any particular case.

When using SMAW, the steel electrode (ESt) will give a very hard weld, non machinable, useful only for very small repairs.

Standard low hydrogen electrodes like E7018 have been used successfully, provided they were dried thoroughly to minimize moisture content. Even iron powder containing electrodes like E7024 were employed with good results.

Cast Iron electrodes (E-CI) with about 2.0%C, provide a structure similar to that of gray cast iron: the weld metal is likely to harden unless proper provisions are put in place.

For difficult cases conducive to cracks, using Ni-Fe electrodes (ENiFe-CI or ENiFe-CIA) with about 50 %Ni- 50% Fe is probably the best selection for the dissimilar welding of cast iron to mild steel, although not the most economic.

If a more ductile or machinable weld must be obtained, high nickel electrodes (more expensive) can be tried, like ENi-CI or ENi-CIA, that will result in a soft, ductile and machinable deposit.

If a more strong weld is needed, for example for nodular irons of elevated mechanical properties, ENiFeMn-CI can be used, where the addition of manganese improves strength, ductility and resistance to cracks.

The electrodes included in the AWS Specification A5.15 are not the only ones available. Proprietary electrodes, not classified with AWS, are available with improved properties for special applications. In difficult cases it may be worth to seek advice from electrode manufacturers, to experiment and to check results.

For certain production lines, higher deposition rates than those available by SMAW (with covered electrodes) may be preferable. In these cases GMAW (Mig) has been applied successfully, especially for ductile or malleable iron.

The wire composition is similar to that used with covered electrodes. Steel wires of types ER70S-3 and ER70S-6 have been used and also ERNiFe-CI. Also nickel containing wires (high nickel, nickel-iron, nickel-iron-manganese) are being used. All other precautions should be in place as necessary.

This presentation would not be complete without including also the solutions that employ the oxyacetylene flame. The slower heating rate of this process causes a larger heat affected zone to form but effectively avoids the development of brittle martensitic structure. Consumables are cast rods with higher levels of carbon and silicon than the castings.

RCI, RCI-A and RCI-B are used respectively for gray cast iron, higher strength alloyed iron, and for malleable and ductile iron. Suitable fluxes must be used to protect the molten metal from oxidation. Preheating must be provided. Slow cooling must be ensured.

Besides welding, in certain cases it would be useful to consider also braze- welding as a possible solution. See Braze Welding.
Here the filler metal is copper base and the cast iron is not melted. Less heat, less distortion, less cracking, machinable filler metal and generally adequate strength is provided. The most serious difficulty, that may sometime prevent its adoption, is the color mismatch of the braze-welded joint.

4 - Filler Metals for Magnesium Alloys

The ASTM Magnesium alloy Specifications governing designations and chemical composition of castings and wrought products are given in our webpage. Also the AWS welding filler metal Specification is reported there. Click on Magnesium Welding.

The most applied process used for repair of Magnesium Castings is probably GTAW (Tig). It is suitable for short weld repair beads with proper filler metal rods.

Pure Tungsten (EWP), zirconiated (EWZr) or thoriated (EWTh-2) electrodes are used. Alternating current with superimposed high frequency current or direct current with electrode positive is used for Magnesium welding.

Filler metal ER AZ61A is used for welding the wrought materials (sheet, extrusion) listed hereafter:
AZ10A, AZ31B, AZ31C, AZCOML, AZ61A, AZ80A, ZE10A, ZK21A

The following Table indicates the filler metals used for welding the designated casting alloys:

Table
Filler Metal for Welding
of Cast Magnesium Alloys

Filler Metal	Casting Alloys
ER AZ101A	AZ92A-T6, for Al containing Cast Alloys, for joining HK31A, HZ32A to any other casting alloy
ER AZ92A	AZ91C-T6, AZ92A-T6, for Al containing Cast Alloys
ER EZ33A	EZ33A, HK31A, HZ32A to themselves or to each other

The other process, used when higher weld deposition rate is required, is GMAW (Mig). The transfer modes accepted are short circuit, pulsed arc, and spray transfer. Globular transfer should be avoided. The transfer mode has to be selected according to the thickness to be welded.

Direct current with Electrode Positive (DCEP) (reverse polarity) is used. Constant Voltage power supplies are preferred. After welding, heat treated castings are often heat treated again or at least stress relieved to prevent stress corrosion cracking in service. Time at temperature should be kept short to avoid grain growth.

5 - Online Press: recent Welding related Articles

From Kodak
Learn how nondestructive organizations introduce new technology, from a paper by Kodak, titled "Transitioning to Digital Radiology", that is available for no-cost download on your computer after subscribing.
http://www.kodak.com/eknec/PageQuerier.jhtml?pq-path=2608/2611&pq-locale=en_US&CMP=OTC-go-ndtproducts

From AWS
Guidelines for Laser Welding of Sheet Metal
http://www.aws.org/w/s/wj/2005/08/023/

From the Fabricator
Safety and Health
http://www.thefabricator.com/Articles/Safety_Exclusive.cfm?ID=1126

From TWI
Weldability of Cast Iron
http://www.twi.co.uk/j32k/protected/band_3/jk25.html

From NIST
Handbook 150 on Procedures and Requirements for
Laboratory Accreditation
http://ts.nist.gov/ts/htdocs/210/214/docs/final-hb150-2001.pdf

6 - Terms and Definitions Reminder

Accreditation means approval, released by a recognized authority, that certain services, including testing laboratories, inspection organizations and reference material producers, have been assessed against international standards to demonstrate satisfactory competence, impartiality and performance.

Cast (referred to GMAW), is the diameter of a single loop of wire, cut from the spool and laid unrestrained on a flat surface. It is a specification requirement that has to be verified at receiving inspection. Out of tolerance cast can cause wire-feeding problems in the wire feeder and contact tip, which will cause adverse effects on arc stability.

Conversion coating is a compound of the surface metal, produced by chemical or electrochemical treatments of the metal. These are chromate coatings on zinc, cadmium, magnesium, and aluminum, and oxide and phosphate coatings on steel.

Helix (referred to GMAW), is the vertical distance between the ends of a single loop of wire, cut from the spool and laid unrestrained on a flat surface. It is a specification requirement that has to be verified at receiving inspection. Out of tolerance helix will produce rotation of the wire in the feeder and contact tip, besides wandering of the wire end.

Non sparking or spark resistant tools are made of metals such as brass, bronze, Monel metal (copper-nickel alloy), copper-aluminum. They are preferred for use in explosive environments, but extreme attention is anyhow required to avoid ignition.

Overheating means heating a metal or alloy to such a high temperature that its properties are impaired. If the original properties cannot be restored by further treatments, the overheating is also known as burning.

Polishing is a preparation technique for metallographic specimens using mechanical, chemical, or electrolytic processes to obtain a smooth, reflective surface suitable for microstructural examination. Deformation artifacts or damage introduced during prior sectioning or grinding should be completely removed.

Weld pass consists in a single run of a welding operation along a joint, weld deposit, or substrate. The weld metal deposited in one trip along the axis of a weld is called a weld bead.

7 - Article: Joining Composites to Metal

With the constant development of exceptional performance composite structures made of strong fibers in a matrix of organic materials, the question of adequate joining technology to metallic elements presented itself early in time as a critical subject of research.

Lightweight composite materials have high strength-to-weight and stiffness-to-weight ratios. Therefore these materials are excellent replacements for metals, provided useful joining methods are developed for composite to metal transition.

Adhesive bonding has been widely used for joining of composites because these bonded joints improve fatigue life, reduce weight of structures, and transfer stresses homogeneously.

Bonded parts generally have mechanical properties lower than base materials because of interfacial or transitional zones displaying quite different chemical and microstructural properties.

Moreover, between the different materials, there may be substantial differences in the coefficient of thermal expansion (CTE) that introduce differential thermal stresses. Sensitivity of adhesives to moisture content can reduce strength appreciably.

Fatigue damage of the adhesively bonded composite-metal joints appears to occur in the form of delamination at the interface.

In general one tends to design transition joints with gradual overlap (scarf or stepped lap joint) where the tensile force (parallel to the bonded surfaces) translates in shear stresses in the adhesive joint.

Carbon-epoxy composite elements are currently adhesive joined to titanium for aircraft primary structures with spliced joints. Bonding with this method provides substantial weight savings without stress concentration at holes (needed for fasteners).

Lately a new development presenting special sculpted protrusions formed on the metal surface by electron beam technique has been proposed to provide more effective resistance to the operating forces.

Issues regarding the type of organic matrix of the composite, either thermosetting or thermoplastic, as well as the curing temperature required for joining, without impairing the composite structure, must be addressed before selecting the actual joining adhesive to be used.

Original manufacturing operations are conducted using all the needed facilities including autoclaves. For repair operations to be conducted in the field, the selection of suitable methods is much more limited, although satisfactory techniques have been developed, where adhesive can be cured with a heating blanket and a vacuum bag.

8 - Site Updating

Our new Page of the Month deals with a Lilliputian world of thin or tiny components, where welding must be carried out, but with care, under a microscope, not to burn through the whole thing.

We are talking of Micro Welding, a loose term still not well defined, that is roughly applicable to a wide array of different processes and means, each one developed for specific needs.

Miniature torches and special power supplies for arc welding or mini machines for spot welding are suitable for manual work. Laser micro weld can be adapted to automatic mass production.

See the new page: click on Micro Welding.

For an updated index of the whole Website click on the Site Map.

9 - Short Items

9.1 - Atomization is a set of processes intended to reduce materials to very fine size (ideally atomic size), by dispersion of a liquid by a rapidly moving gas at high pressure through a fine nozzle. Alternatively in rotary atomizers the liquid is continuously accelerated to the wheel edge by centrifugal forces, produced by the rotation of the wheel and then discharged at high speed.

Molten metals are atomized under a protective atmosphere and then quenched in a cold liquid or on the surface of a cooled slowing rotating cylinder. The collected powders must be separated according to size before they can be used for further processing.

9.2 - Diffraction is the apparent bending and spreading of waves when they meet an obstruction. Diffraction is one type of wave interference, caused by the partial blocking or lateral restriction of a wave. As applied to the study of metallic materials, x-ray diffraction permits to deduce the internal arrangement of atoms in crystals, presenting a periodic, ordered structure, from the interference patterns displayed on a recording medium (e.g. film) by a beam of x-rays that propagate as waves.

9.3 - Materialography is an extension of metallographic techniques, used to examine duly prepared specimens under the microscope for their description and characterization. After the evolution of mineralographic and petrographic studies, ceramographic techniques were developed for the study of engineering ceramics. With the further diffusion of composite materials, similar techniques had to be improved and adapted to the new materials. Nano materials are now being studied most intensively. The science and technology of the examination of materials uses similar equipment and means for preparing the specimens for further study. Materialography is a new name that groups all of the similar techniques of cutting, mounting, grinding, polishing and etching specimens for microscopic examination independent of the nature of the specific material.

9.4 - Quantitative Metallography is a specialized discipline that makes use of computer aided picture analysis of visible features appearing under the microscope (and projected on a screen) on the surface of prepared specimens. By recognizing, grouping and performing simple geometric calculations on certain distinctive characteristics of the examined specimen surface, the technique permits to quantify their appearance and frequency. By these means the size, the aspect ratio and the amount of one or more phases in a given section can be expressed as a fraction of the matrix area. The correlation of the relative geometric properties as observed in metallographic sections, to the physical behavior revealed by testing, permits to perform studies and to use Quantitative Metallography as a tool for quality control.

9.5 - Scanning Electron Microscope (SEM) is an instrument that forms an image on a Cathode Ray Tube (CRT) (similar to a TV or Computer Screen). The scan or raster of the screen is synchronized with that of a very fine beam of electrons that scans the surface of the specimen in a vacuum chamber. The brightness of any point of the image is proportional to the quantity of electrons scattered from the specimen and collected on specially designed antennas. The intensity of emission of both secondary and backscattered electrons is very sensitive to topographical features on the specimen, that are rendered in an intuitive way in the displayed image.

The magnification produced by scanning microscope is in the range of between 10 to 200,000 X and the resolution (resolving power) is between 4 to 10 nanometer (1 nm = 1 * 10^-9 m)

9.6 - Stress Corrosion Cracking (SCC) is the formation of brittle cracks induced from the combined simultaneous influence of tensile stress and a corrosive environment.

In some cases, crack propagation is along grain boundaries, in other cases, the path is along specific crystallographic planes within the grains. SCC is strongly affected by alloy composition, the concentration of specific corrodent species (i.e. chlorides) and by the stress intensity. Chloride induced cracking of stainless steels, caustic cracking of plain carbon steels and ammonia damage to copper alloys are typical examples of this problem.

Localized corrosion can promote SCC. Therefore exposure geometry and specimen design are important factors. Besides that, to oppose SCC one has to select alloys resisting to this form of corrosion and to reduce the residual stresses to the minimum by suitable stress relieving processes.

For a comprehensive discussion of SCC see:
http://www.npl.co.uk/ncs/docs/stress.pdf

10 - Explorations: beyond the Welder

On The Growing Urbanization of the World
http://www.earth.columbia.edu/news/2005/story03-07-05.html

If you want to send your voice into intergalactic space, here is how:
http://www.talktoaliens.com/

Many References on Nanotechnology at
http://www.zyvex.com/nano/

On development of diagnostic tests for infectious diseases for developing countries
http://www.finddiagnostics.org/index.htm

International Development Enterprises on Fighting Rural Poverty:
http://www.ide-international.org/

11 - Contribution: Thermal Cutting Processes with secondary Water

Two articles in the recent Issue 4 of Welding and Cutting present the case for water assisted Thermal Cutting Processes. The first one, at page 191, compares performance and costs of different processes including, besides plasma in air and with water injection, also laser and oxyacetylene cutting.

Underwater plasma cutting is credited with lower dust emission and reduced noise, along with lower distortion due to the additional cooling of the cut plates. The most disturbing disadvantage is the need for a complicated water treatment for collection of powders and disposal thereof.

The other article, at page 200, deals with Contact Arc Metal Cutting under water, a process particularly suitable for dismantling nuclear installations. The surface finish obtained is quite rough, the speed achievable is good, but the graphite electrodes wear rapidly, and replacement rate is important. Studies were performed to assess the influence of the water mass flow on the life of the electrodes and on process efficiency.

Interested readers are urged to seek the original articles.
For receiving two issues of Welding and Cutting free of charge, without obligation, possibly in view of a regular subscription, you can navigate to
http://www.welding-and-cutting.info/

12 - Testimonials

From: selva kumar 'metselva@yahoo.com'
To: Welding Advisers
Date: 02 Aug 2005, 01:52:14 PM
Subject: Re: HF induction welding

Dear Elia Levi
Greetings, Very useful article.
Thank you.

Regards,
L.C.Selva Kumar

From: jeffrey leo 'jeffreyhans1@yahoo.com'
To: Welding Advisers
Date: 16 Aug 2005, 02:23:45 PM
Subject: Re: Arc striking

Hi,
Thanks for the information. I have learned a lot from it
and now will be able to do the job. I get it.

13 - Correspondence: a few Comments

In our Form for Questions and Feedback we introduced recently two new items that we request our correspondents to fill in. It is just appropriate that we should know who is writing. We are not requesting too much, only the Responsibility the inquirer has and a description of the Organization one works for. We feel this is important in order to have an idea of the public we are talking to.

Unfortunately sometimes people just skip these questions.
We do not agree. Please fill in all the blanks. We will try to have inquirers complete the form before formulating our answer.

14 - Bulletin Board

14.1 - ASNT Fall Conference & Quality Testing Show 2005
Columbus, Ohio, 17 - 21 October 2005.
www.asnt.org

14.2 - The ASM Thermal Spray Society (TSS) announces the Combustion Turbine Coatings Symposium 2005, to be held Oct. 26-27 in Houston, Texas.
www.asminternational.org

14.3 - If you were confused by too many offers and did not yet grab the opportunity of downloading at no cost your copy of the highly respected book Affiliate Masters Course offered by our Web Host firm SiteSell, you should do it now. Click on your copy of Affiliate Masters Course.

See you next time

Copyright (c) 2005, by Elia E. Levi and welding-advisers.com, all rights reserved