Oct 31 25 2002.
Author Ed Craig.
This is a partial report.

THE FAILURE OF WATER WALL BOILER TUBES CAN COST USA POWER PRODUCERS MORE THAN FIVE BILLION DOLLARS ANNUALY, YET MINIMAL ENGINEERING EMPHASIS IS OFTEN PLACED ON THE SUITABILITY OF THE CLADD WELD PROCESS AND CONSUMABLES UTILIZED.

WELD CLADDING WATER WALL BOILER TUBES,
WELD PROBLEMS AND UNIQUE SOLUTIONS:

Boiler tube weld cladding, can create dramatic cost savings for power companies, however for the practical weld decision maker that deals with the subject of weld cladding boiler tubes, the common weld process control logic applied may sometimes appear as dense as the weld smoke that surrounds the process.

This weld article examines some of the pulsed MIG, clad weld issues that occurred on USA, manufactured, boiler walls. The welding occurred during 2002. An evaluation was also carried out on a unique MIG weld process that in contrast to the pulsed MIG mode can provide numerous weld benefits for cladding and weld surfacing applications.

The traditional North American approach to clad welding water wall tubes is to utilize Japanese Pulsed MIG equipment MIG, along with an argon or argon helium gas mix, and 1.2 mm, nickel, chrome molly MIG wires such as 622 or 625.

When factory cladding new boiler wall tubes, the mechanized, continuous, clad welds may be made in the flat position on a single tube that rotates.

Mechanized, single pass, MIG clad weld made on a rotated
water wall boiler tube.

 

When welding one complete side of a new boiler wall panel, mechanized, overlapping linear welds are made in the flat welding position. For on site boiler clad rebuilds, or clad weld repairs, the clad welds are typically carried out by field weld crews who utilize portable, mechanized equipment and the vertical down weld technique for the tube clad welds.

The following photo is a shot of mechanized, overlapping, linear pulsed MIG clad welds. Note the extensive manual TIG weld repairs required on the tubes cladd by GE. The poor welding results from the pulsed process along with the negative attributes of the high enerdy TIG process will have extensive repercussions for the boiler manufacturer and the sub contracter who did the clad welds.

Pulsed MIG alloy 622, linear clad welds made by GE on one side
of the boiler panels. The circles indicate a few of the many
areas that required extensive TIG repairs.

 

The pulsed MIG process is used in North America for boiler wall cladding and numerous other weld surface applications. The pulsed MIG process is compared in this article with a unique, propriety, none pulsed MIG process, called the Zue's Process, (ZP). The US, Zues Process was developed to optimize the MIG welding results attained with high alloy consumables that are utilized for joining, build up, cladding, hard facing and weld casting applications.

Evaluation of an Actual Boiler Water Wall Tube, Clad Weld Application:

In North America there are a handful of ASME approved companies who specialize in boiler tube cladding. When a boiler manufacturer requires weld cladding on its low carbon, low alloy steel boiler tubes or panels it's not unusual for that manufacturer to sub contract the weld cladding operation. All the data in this report is based on an actual weld cladding operation carried out in the USA during 2002. General Electric provided the cladding on the water wall boiler tubes..

On the application discussed in this report GE used an 0.045, (1.2mm), alloy 622 MIG wire, with an argon 25 - 30% helium gas mix. Both the 622 and 625 MIG alloy wires utilized for water wall cladding are similar from a chemistry and welding perspective. With the pulsed MIG process, both alloys typically produce sluggish welds which can create unique clad weld issues as discussed in this report.

Before welding commences on the water wall tubes, GE first sandblast the carbon steel tube surface. To minimize the weld heat generated by the continuous clad, pulsed welds, the boiler tubes are typically filled with running water.

These water wall tubes were pulsed MIG welded as the tubes were continuously rotated. Later the tubes were bent. During the bending operation surface weld cracks became evident in the clad weld surface. The small cracks occurred due to the poor, pulsed MIG weld tie-ins. To compensate for the sluggish MIG welds on the bent tubes, GE engineers made a poor weld decision and added a TIG wash weld to the pulsed MIG weld surface.

 

 

MIG / TIG Wash Welds on Formed Boiler Tubes :

Note: The boiler manufacturer, and power company weld inspectors and engineers were very pleased with the smooth, weld surface cosmetic appearance of the TIG washed, pulsed MIG welds provided by GE, (THE BLIND LEADING THE BLIND) . Unfortunately the inspectors and engineers were not made aware, that on these clad tubes, the TIG wash had negatively influenced the clad weld chemistry and clad thickness. These welds no longer met the power companies contractual, clad weld specifications and these out of spec clad welds ended up in the new boilers.

WITH CLAD WELDING, IT'S WHAT THE INSPECTORS AND ENGINEERS DON'T SEE THAT SHOULD ALWAYS BE A MAJOR CONCERN.

Clad welding issues are influenced by many factors;

[1] In the power / boiler industry it's not uncommon to find that the engineers who are given responsibility for the pulsed MIG clad welds, have in reality, minimal pulsed MIG weld process control expertise.

[2] In the power industry it's not uncommon to find that the clad weld specifications written by the utility company or boiler manufacturer engineers do not fully address the fundamental requirements for evaluating the welds after the cladding operation is complete or after weld process changes were made.

[3] In the power industry one of the most common welding processes utilized is the TIG process. While the high energy, TIG process is ideal for attaining good weld penetration on pipe and tube applications, this popular process is rarely suited to the unique, low dilution, low weld energy requirements of a clad weld. On the application discussed in this report, neither the engineers who designed and built the boilers or the sub contractor who clad the tubes gave consideration to the negative impact of the TIG process as used for clad weld repairs, or for the surface weld wash on the pulsed MIG welds.

A CLAD WELD BOILER WALL IS ONLY AS GOOD AS THE
WEAKEST LINK IN THE CLAD WELD.

THE TIG PROCESS AND THE PULSED MIG CLAD WELDS.
The cladding company found with it's pulsed MIG process, that no matter how well tuned the pulsed MIG welding parameters were, the alloy 622 pulsed MIG weld fusion at the clad weld tie-ins was both insufficient and inconsistent. This became evident were the clad tubes were bent and surface weld cracks were found with dye penetrant.

To overcome the circumferential, pulsed MIG, clad weld cracking issues that occurred with the bent tubes, GE, a highly experienced cladding company, a company dedicated to delivering clad welds with "controlled chemistry and depth" believed the logical solution was to use a 400 to 600 amp TIG wash on the clad weld surface and blend together the pulsed MIG weld passes.


WITH BOILER WALL WELD CLADDING, A POWER COMPANY WILL PAY A PREMIUM FOR WELD PROCESS IGNORANCE AND ANOTHER PREMIUM FOR THE HIGH ALLOY CLAD WELDS. THE WELD REALITY IS THIS. THE CLAD WELDS RECIEVED BY THE POWER COMPANY MAY HAVE LITTLE IN COMMOM WITH THE WELD SPECIFICATIONS THEY PROVIDED OR WITH THE ALLOY CONTENT OF COSTLY WELDING WIRES THEY PURCHASED.

The following macro photo shows the influence of the worlds best Pulsed MIG power source. This weld section reveals the pulsed MIG, welds before the TIG wash. Note; the high weld heat and deep, scalloped inconsistent weld fusion. This weld fusion and profile can result in high weld stress in the tubes and excess weld dilution with the alloys in the single pass clad welds. Examine the size and coloration of the heat affected zone (HAZ). A HAZ with martensitic formation. Note; also the scalloped, inconsistent weld surface.

PULSED MIG CLAD WELD BEFORE THE TIG WASH.

 

The next macro reveals the influence of the "TIG wash" on the 622, pulsed MIG clad surface. Note; how the inconsistent weld surface is smoothed out, an effect that won approval from GE , the boiler manufacturer and utility company engineers. Note none of the engineers requested an evaluation of the clad welds after the MIG process and TIG wash. What the engineers did not see was how the clad weld thickness was decreased and was ouside the weld spec requirements. What the engineers did not see was how the pulsed MIG weld fusion was increased. The extra heat from TIG was sufficient to smooth the internal weld fusion line and the HAZ size and weld dilution were both dramatically increased.

The TIG wash completely re-melted the pulsed MIG clad weld, the clad weld thickness and clad alloys were reduced outside the weld specification requirements and the martensite (hardness) formation was increased.

PULSED MIG CLAD WELD AFTER THE TIG WASH.

THE ZUE'S PROCESS:
In the following macro, compare the Zue's weld process against the pulsed MIG welds. The circumferential 622 welds made with the ZP required no sand blast or weld surface preparation and required no TIG wash.

Note with the ZP, the consistent, yet minimal weld fusion attained, (<0.007), also the lack of scalloped weld surface and weld fusion. Metallurgist should also have an interest in the lack of an evident heat affected zone. With the ZP there was no evidence of an increase in the HAZ tube hardness and no martensite was found.

In contrast to a pulsed MIG weld, the ZP will produce much less weld heat input, and in many single pass weld overlay applications, as a result of the reduced weld fusion potential (less weld dilution) the ZP will require less filler metal and less weld passes. The ZP greatly reduces the need for traditional weld pre-heat or post heat treatment on alloy applications, and also eliminates the need for sand blasting or special surface preparation.

Engineers when dealing with single pass weld overlay applications should always be focused on the weld heat and weld dilution potential. However when the ZP is applied, the engineers will know that in contrast to the traditional pulsed MIG mode, the ZP can often provide in a single weld pass, an alloy chemistry equivalent of two or more layers of a typical pulsed MIG clad weld.

WITH A PULSED MIG WELDS YOU MAY HAVE TO DEPOSIT 40%
MORE CLAD WELD METAL.

The minimum clad weld thickness as specified for the water wall tubes was 0.070". With the GE pulsed MIG welds, the clad weld thickness started out at approximately 0.120. After the TIG wash or TIG repairs the clad thickness was found to be as thin as 0.050.

In contrast with the lower weld fusion Zues Process, to attain a clad thickness of 0.070 you would typically have to apply a weld pass with only 0.080.

I did random tests on the GE burner ring clad welds. The tube cut sections revealed in the bend sections, that several clad weld locations had a clad weld thickness less than the specified 0.070 and as thin as 0.050.

The GE clad weld thickness had been influenced by both the tube bending action and the thinning action of the TIG wash.

 

 

This TIG washed, pulsed MIG, clad weld made by GE was supposed
to provide a minimum clad wall thickness of 0.070.

.

As the clad welds discussed in this report were made by GE in a "controlled factory environment", I often wonder how much consideration is given by the utility or boiler companies to the clad weld integrity and chemistry of the on site clad welds made inside their boilers? On-site clad welds are typically made on worn, thin tubes of variable wall thickness. Tubes with variable wall thickness will provide a variable weld heat sink, influencing the iron and alloy dilution between the single pass clad welds and the low alloy, carbon steel tubes. Special weld process control consideration is required for on site clad welds to ensure the weld procedures utilized achieve the chemistry and thickness goals.

Often to provide the optimum alloy composition for the clad welds, the only solution with the pulsed MIG process is to provide two clad weld passes. However its been my experience that few managers or engineers in the power industry have an interest in the subject of weld cladding. This engineering apathy results in many clad welded boilers ending up with a $10 a pound, single pass clad welds with a composition that in reality offers no more protection than what would have been attained if the cladding was welded with a regular $1 a pound carbon steel consumable.

CLAD WELD ALLOY CONTENT:

The chrome content of the 622 or 625 MIG clad weld consumables is considered the prime alloy for weld corrosion protection. The level of the chrome content desired in the clad welds is of course influenced by the weld process and application considerations.

The boiler manufacturer wanted GE to supply a "single pass", 622 clad weld that contained >19% chrome. This is ironic as the weld consumable selected contained only 20.5% chrome, as indicated in table one. The cladding company stated they could only guarantee a minimum of 18% chrome for the single pass, 622 clad welds made on the low alloy, carbon steel boiler tubes.

Table.1.

Alloy 625 PrimaryAlloys	Alloy 625 Approx Alloy Percent %	Alloy 622 Primary Alloys	Alloy 622 Approx Alloy Percent %
Chrome	21.5	Chrome	20.5
Iron	2-5	Iron	2-3
Nickel	58	Nickel	58
Moly	9	Moly	14

 

Note: With the alloy 622 weld wire, the chrome and iron content in the wire before the weld dilution occurs with the carbon steel boiler tubes. The GE clad weld chemistry samples in this report were randomly taken from the actual boiler parts being made for the mid west utility company. The initial chemistry tests of the clad chrome surface were first taken with x-ray analysis. Later more extensive and controlled laboratory tests were carried out to confirm the clad chemistry results. Tests were also carried out to evaluate the clad weld chemistry throughout the clad weld depth.

The GE cladding company weld procedure qualification" for the clad welds provided a single pass weld chemistry analysis of, Cr >18 % - Moly >12. % - Ni >51 % - Iron <12 %.

This "minimum" weld alloy content was verified and approved by the boiler manufacturer at the start of the contract, (I am sure this alloy content was not taken from the welds that had undergone a TIG wash).

Due to the clad weld surface micro crack issues that arose, GE weld personnel decided that they should "increase the TIG wash weld current". The increase in the TIG wash amps was approved by the boiler company's engineers who again failed to follow up and test the cladding after the weld changes were made.

The actual, 622 clad weld chemistry measured on the water wall tube sections removed from the boiler components revealed the following. While the Zues Process weld alloy content was within the specifications.

 

 

Wrought Alloy 622 MIG wire, and GE - Zues Clad Weld Data:

Alloy 622 primary MIG weld alloy	Alloy 622. Approx. alloy percent in MIG wire	Alloy 622. Company GE The minimum alloy content specified in the boiler clad welding specification	Alloy 622. Company GE Actual alloy content found in pulsed MIG / TIG wash welds produced by cladding company. Single pass weld.	Alloy 622. Zues Process Actual alloy content found in Zue MIG single pass weld. This range is from the cladd weld surface to where the clad contacts the carbon steel boiler tube
Chrome	20.5	>18	13.7 to 17%	18 - 19%
Iron	2-3	<12	20 - 30%	7.2 - 8.2%
Nickel	58	>51	32 - 45%	54 - 58%
Moly	14	>12	8 to 11.5%	12 to 14%

 

 

A Macro of a ZP Circumferential 622 Clad Weld:



Note: With 7 weld passes per inch on the rotated tube, note the
relatively smooth, untouched weld surface. The minimum yet
consistent weld fusion, and the lack of a noticeable HAZ.
With the ZP, single pass, clad welds, the majority of the
costly alloys end up in the weld, not diluted in iron.

CLAD WELDING PROCESS COMPARISON.
In contrast to the traditional pulsed MIG process;

[a] The Zues Process did not require that the boiler tubes surface be sand blasted.

[b] The Zues Process provided a clad weld that was less penetrating, less dilution resulting in greater alloy retention in the weld.

[c] The Zues Process resulted in a single pass weld that provided "greater total wall thickness". In water wall cladding, less clad "weld penetration" means "increased wall thickness",

[d] The Zues Process minimal weld fusion weld typically will require 20 to 40 much less filler metal.

WITH THE ZP, LESS IS MORE. LESS WELD DILUTION EQUALS MORE WALL THICKNESS.

[e] The Zues Pocess produces much less weld heat yet reveals no sluggishness in the welds. This results in superior clad weld tie- ins with less weld stresses and part distortion.

[f] The GE pulsed MIG, process produced martensite in the water wall tube heat affected zone (HAZ). The Zues process produced a weld in which no increase in hardness was evident.

[I] For those alloy cladding applications that traditionally require pre and post heat treatment, the low heat input ZP will on numerous applications eliminate these requirements.

In conclusion.

It's evident from both the weld and chemistry evaluation of the traditional pulsed MIG process and the Zues Process, that the ZP delivers a very different clad weld, a weld that can provide many cost effective feature benefits. There is not much logic for a boiler or power company to pay to $12 a pound for a MIG clad weld wire that due to the weld process used may produce a weld which contains 10 to 30 % iron.

There are alternative methods to "pulsed MIG" for clad welding and the USA developed Zue's Process can provide many weld quality benefits. From a weld cost perspective the ZP offers the opportunity for dramatic cost reduction for cladding or weld surface applications.

[1] As much less filler metal will be required, a cladding company could anticipate an extensive reduction in the alloy filler metal costs.

[2] As the pulsed process puts extensive heat into the boiler tubes, when welding the pulsed application is restricted to a single MIG gun per boiler tube. In contrast, the ZP puts in so little heat that 2 to 4 air cooled weld torches could be used on a single tube. This has the potential to reduce the weld cladding labor costs by 50 to 80%.

This report pointed out a few of the issues that can be found today with the cladding process and the boiler industry. There were two fundamental reasons that the US manufactured boiler panels mentioned in this report are now installed in boilers and have inferior clad weld quality,

[1] A blinker, egotistical, bury your head in the sand mentality by the power industry engineers responsible.

[2] Weld process ignorance from all the parties involved in this boiler contract. The most difficult thing to implicate in an industry embedded in tradition, is "change". If engineers in the power, chemical and oil industry could see past the pulsed process and learn a little more about the MIG process, they would find great opportunities for weld surface weld quality improvements and weld cost reductions.