M. P. Dhanuka, Executive Director (Marketing), General Electrodes and Equipments (GEE) Ltd.
Electric arc welding has been in use in industry for a very long time. Because of its versatility, it appears that this process will dominate the field of joining metals and alloys for many more years. The first steps in electric arc welding were taken by Bernados and Olszewaki in 1885 when they patented a method of welding, using an arc struck between a carbon electrode and the workpiece. The direct use of a metal-arc electrode was patented in the U.K. by N. Slavianoff in 1890. The first reference to a welding application in the ‘Journal of the Iron & Steel Institute’ appears in 1901 and describes the welding of cast iron and wrought iron at Furstenwald near Berlin using the Slavianoff process and a flux of powdered glass plus 1% of ferromanganese. The first patent for covered electrode was filed in the year 1907 by O. Kjellberg, followed by another patent in the year 1912. Strohmenger in 1911 patented an electrode wound with asbestos yarn impregnated with sodium silicate. The first all-welded barge was launched in June 1918. The first version of basic type of covered electrode was developed during the early ’30s. The covering of the electrode had a high proportion of lime and fluorspar. In Germany, basic type of electrodes was used during World War II for welding of armour plates as an effective substitute for the austenitic types. One of the most successful modern rutile-based general purpose electrode was first developed in 1941 for Welding in shipyards. It is still one of the largest selling electrode in most of the countries. During the 30’s, 40’s and 50’s, a large number of electrodes were developed depending on application requirements. Based on Flux coating formulations, these electrodes can be classified broadly as under:
|TYPE OF COVERING||SFA 5.1 CLASSIFICATIONS|
|Cellulosic coated electrode||E6010,E6011|
|Acid type -High iron oxide with
or without iron powder
|Rutile, Rutile cellulosic type
electrode with or without
|Low hydrogen electrode with
or without iron powder in the flux
|Iron oxide with titania||E6019|
Achieving high quality welds
Achieving high quality welds is the objective of every fabricator and it is the basic requirement of his client. Deposited weld metal can be termed as high quality weld provided the weld metal fulfills the following requirements.
- Deposited weld is free from defects.
- Deposited weld has required performance.
- Deposited weld meet required standards and specifications.
Types of electrodes and mechanical properties of the deposited weld metal
Various types of electrodes conforming to respective AWS/SFA classifications fulfill the code requirements with respect to chemical composition of weld metal, mechanical properties of weld metal, and usability and soundness test requirements.
Table 2 gives details of impact test requirements for various types of electrodes.
|AWS / SFA CLASSIFCATION||CHARPY V-NOTCH IMPACT
|6012,E6013,E6020,E6022, E7014,E7024||Not specified|
Hydrogen content of the weld metal and types of electrodes:
Only Low-Hydrogen basic type of electrodes such as E6018, E7015, E7016, E7018, E7048, E7028, E7016-1, E7018-1 and E7018M have been designed to deposit weld metal having low levels of hydrogen. Hydrogen content in the deposited weld metal depends on many factors. These aspects will be dealt with separately in this paper.
Hydrogen in the weld metal and susceptibility to cold cracking in ferritic welds
Defects in welds
Weld defects are broadly classified into two types:
- Defects induced by the welder due to low skills, difficult Working environment etc.
- Defects due to the metallurgical characteristics of the material.
|Defects due to welder||Defects due to Metallurgical Characteristics|
|Lack of side wall fusion||Hydrogen cracking|
|Lack of penetration||Solidification cracking|
|Undercutting at toes||Lamellar tearing|
Hydrogen induced cracking in welds
Hydrogen cracking in the weld metal as well as in the parent metal has been the subject of intense research for many years. However, it is still the most common defect found in welded structures. The cost of repairs due to hydrogen cracking is, on a worldwide basis, enormous. Traditionally, hydrogen cracking occurs in the hardened coarse grained HAZ region of ferritic steels but the advent of low carbon steels has seen a significant increase in weld metal hydrogen cracking. HAZ Hydrogen cracking occurs due to four factors which are as under:
- Susceptible microstructure in the coarse grained HAZ
- Ambient temperature
If anyone of the above factors is removed, then it would be possible to prevent the cracking of the weld metal or HAZ cracking Even though all the 4 factors mentioned above have equal importance, hydrogen is an elusive element which affects cold cracking susceptibility in ferritic welds. Hence, control of hydrogen in the weld metal is of paramount importance.
Hydrogen in the weld metal can be introduced from a number of sources. The most likely sources are as under:
- Moisture in the electrode coating or moisture in fluxes
- Moisture on the steel surface
- Water combined with rust, oils and other greases on the work piece or filler wire
- Organic degreasing agents used for cleaning
- Moisture present in the atmosphere
- Residual hydrogen in the steel- especially in thick section Material
In the case of manual metal arc welding, the main source of hydrogen is the moisture contained in the electrode coating. During welding, dissociation occurs and the atomic hydrogen thus formed dissolves in the molten pool. During cooling much of the hydrogen escapes from the solidified bead by diffusion but some quantity diffuses into the HAZ and the parent metal. The amount which does so depends on several factors such as the original amount absorbed, the size of the weld, the decreasing solubility and time temperature conditions of cooling.
Amount of hydrogen absorbed by molten weld pool varies with concentration in atmosphere surrounding the arc. Solubility at 1,900°C.
Solubility of hydrogen in weld metal decreases as temperature falls.
In general, the more the hydrogen is present in the metal, the greater the risk of cracking. Control over the hydrogen level of the weld metal may be achieved by minimizing the amount initially absorbed or by ensuring that, sufficient time is allowed to escape by diffusion before the weld cools. Often a combination of both measures provides the best practical solution.
Moisture in the Flux coating of electrodes may be present as absorbed water, loosely combined water of crystallization or more firmly bound molecules in the silicate structure. All these forms can break down to produce hydrogen.
Definition of low hydrogen electrode
Understanding of the role of diffusible hydrogen in delayed cracking of welds in carbon steel and low alloy steel came about at least as long ago as the 1940’s. Indeed, classification of low-alloy steel electrodes designated as “Low hydrogen”, was introduced into AWS specification A5.5-48T (ASTMA316-48T) in 1948 along with a diffusible hydrogen test using collection of hydrogen from a quenched weld bead over glycerine. The Low hydrogen electrode was defined as one meeting a maximum limit of 0.1 cubic centimeter of gas collected per gram of deposited weld metal (10ml/l00g, in units commonly used today for expressing diffusible hydrogen).
Shortly after this specification was published, Stern Kalinsky and Fenton cast doubt on the suitability of collection of hydrogen over glycerine by showing that collection over mercury produced considerably more hydrogen because hydrogen is soluble in glycerine. By way of the 1954 revision of AWS 5.5, the glycerine test was withdrawn from the specification and it was replaced in the 1964 revision by definition of Low-hydrogen, as those having no more than 0.6 percent coating moisture by weight, with lower limits for higher strength electrode. This definition of Low-hydrogen electrodes in terms of 0.6 percent coating moisture became the mandatory section of AWS A5.1 specification in the 1981 revision. AWS A5.1 specification was further revised in the year 1991 and in the year 2004. In this specification, diffusible hydrogen test is required for E7018M electrode, whereas for other Low hydrogen basic coated electrodes, diffusible hydrogen test is only required when diffusible hydrogen designator is added to the classification.
E 7018H4, E 7018H8 and E 7018H16
As per the new classification system for low hydrogen electrodes the upper limit for diffusible-hydrogen is 16ml 100g which fits well with the correlation of 0.6 percent coating moisture. As per AWS/SFA 5.1 specification, diffusible hydrogen content of the weld metal is to be tested in accordance with ANSI/AWS A4.3 specification, “Standard methods for Determination of the diffusible hydrogen content of Martensitic, Bainitic and Ferritic weld metal produced by Arc welding”. As mentioned above, diffusible hydrogen testing is required as per AWS/SFA5.1 specification for all Low hydrogen type electrodes when diffusible hydrogen designator is added to the classification. Diffusible hydrogen limits for weld metal are given in the following table:
Determination of the diffusible hydrogen content of the weld metal by mercury method as against glycerine method
It was recognized long ago that glycerine dissolves not only some hydrogen but it dissolves oxygen, nitrogen, carbon dioxide and water M.A. Quintana showed that, as the glycerine bath aged, the concentration of hydrogen in the gas collected from a diffusible hydrogen test decreases from about 70 percent to about 50 percent As the apparent diffusible hydrogen of the test specimen increased, the concentration of hydrogen in the gas collected over glycerine increased from about 55% at 6ml!100g to about 75% at about 10m 1/1 00 g. The remainder of the gas collected consists of other dissolved gases which are displaced from glycerine by the dissolved hydrogen.
At very low hydrogen level, no gas is collected. All of the hydrogen dissolves without displacing any other dissolved gases. The Japanese have conducted at least three studies of correlation between the glycerine test the IIW method. In the first study, the test weld was quenched 30 seconds after the arc was extinguished. The following correlation equation was obtained.
Hjis = 0.64 Hiiw – 0.93 …….. (1)
In the second study, the test weld was quenched 5 seconds after the arc was extinguished.
The Second study provided slightly higher hydrogen and the equation is given below:
Hjis= 0.67 Hiiw – 0.80 …….. (2)
More recently, the Japanese have moved to adding a run-on and run-off tab to the test specimen. They produced a third equation which is a as under:
Hjis= 0.79Hiiw-1.73 ……… (3)
The three equations are plotted in fig 3 which shows that there is very little difference among them
Correlation between coating moisture and diffusible hydrogen
In evaluating correlation between coating moisture and diffusible hydrogen, it is important to keep in mind that correlation cannot be perfect because several factors can influence the relationship. Few factors which can influence the relationship are as under:
- At the same coating moisture level, an electrode with a heavy coating would deliver to the arc more hydrogen than would an electrode with a thin coating.
- The addition of fluorides to an electrode slag system will cause removal of some hydrogen. As a result, similar electrodes at the same coating moisture level, will yield different diffusible hydrogen levels if one is appreciably higher in fluoride than the other.
HIRAI and co-workers developed a predictable equation which related IIW diffusible hydrogen to as baked coating moisture (a1 percent), moisture picked up by exposure (a2 percent) and partial pressure of water vapour in the air at the time of welding (b, mm of Hg) for E7018 type electrode.
HD- (260al + 30a2 + 0.9b -10) ~ (4)
For freshly baked electrodes, the exposure term of equation (4) could be zero and atmosphere condition for the welding amount to about 10mm Hg partial pressure of water. Based on the results, correlation between diffusible hydrogen versus coating moisture for E7018 is plotted in fig (4).
Effect of basic coated electrode exposure on the diffusible hydrogen content of the weld metal
As-manufactured coating moisture of the basic coated electrode is likely to be more concentrated in the interior of the coating than on the surface because, in baking the electrode, moisture must diffuse outward through the coating thickness in order to escape. Conversely, moisture picked-up during exposure must enter the electrode coating from the outside surface, and diffuse inwards. This surface moisture could be less tightly bound to the coating than the as-manufactured moisture which survived a high temperature baking. In other words, rehydrated moisture picked-up during exposure will be less effective in introducing diffusible hydrogen into the weld metal than the as-manufactured moisture. Dr. Evans provided the results of basic coated electrode diffusible hydrogen versus coating moisture on Drying and on exposure which are given in fig 5.
Correlation of diffusible hydrogen with cracking
Evans and Christense and Christense and Simonsen examined the critical cracking stress by the implant test of a variety of steel as a function of hydrogen content of the weld metal.
An example of their data is shown in fig 6. This data was obtained from a steel of O.17C, 1.36 Mn. Both rutile and basic coated electrodes were used to obtain various hydrogen levels.
The hydrogen levels reported are based upon fused metal (deposited metal plus melted base metal).
Critical cracking stress is plotted in two ways, directly against hydrogen content of the weld metal and against the logarithm of the weld metal hydrogen content via a semi-logarithm plot.
For all steel examined, the critical cracking stress, under otherwise identical conditions, is a linear function of the logarithm of hydrogen content of the weld deposit. It is not a linear function of the hydrogen content. In particular, a small change in hydrogen content is much more important at a low hydrogen level than at a higher level.
At a given preheat temperature, the critical carbon equivalent for crack-free welding varies as a function of the diffusible hydrogen content of the weld metal. Yurioka conducted experiments on the above lines and his result are reproduced in fig 7A & 7 B.
Effect of the atmospheric moisture during welding on the content of diffusible hydrogen in the weld metal
In most of the arc welding processes including shielded metal arc welding, the arc is imperfectly shielded from the air. During the welding process, moisture from the atmosphere does enter the arc and the weld metal.
Ruge and Dickehut developed nomogram for estimating the effect of atmospheric moisture at the time of welding on the weld metal diffusible hydrogen.
The nomogram predicts, for example, that an electrode type E 7018, which produces 3.0ml/l00g. Diffusible hydrogen when tested at 20°C and 40% relative humidity, will produce 5.4ml/l00g. when tested at 30°C and 80% relative humidity, an increase of 2.4ml/l00g.
But an electrode type E 7018 which produces 9.0ml/l00g. When tested at. 20°C and 40% relative humidity, will produce about 10.0rill/l00g. When tested at 30°C and 80% relative humidity, an increase only of 1.0ml/l00g. 8.2
Following conclusions can be drawn from the various experiments carried out:
- The effect of atmospheric moisture at the time of welding is greater for very Low or Ultra low hydrogen electrode than it is for higher hydrogen electrodes.
- Long arc during welding will have more tendency to pick-up moisture from the atmosphere and will produce more hydrogen in the weld metal.
- In GMA and gas-shielded arc welding, drafts or other disturbances to the gas shielding can be expected to increase diffusible hydrogen of the weld metal.
- In self-shielded flux-cored arc welding, high voltage increases the arc length and can be expected to increase diffusible hydrogen.
Welding consumables: Basic coated electrodes
In order to minimise the risk of cold cracking during welding of carbon steel and low-alloy steels, a lot of developments have taken place during the last few decades. Designs of basic coated consumables cover the following four aspects:
- Low, very low and extra low level of hydrogen in the weld metal
- Response to redrying
- Resistance to moisture reabsorption characteristics
- Special Packaging.
By using the most updated in house technology, the most advanced extra low hydrogen type of electrodes having the following classifications have been developed.
These electrodes have the following unmatched properties:
- a) The weldability that WELDERS really enjoy.
Recently developed Low-hydrogen basic coated electrode features the best easy handling characteristics in all positions. The deposited weld metal has uniform ripples and absolutely no spatter. An end to difficult positional welding and lack of fusion on vertical up welding position. These electrodes deposit x-ray/ radiographic quality welds in all positions including 5G, 6G or 6GR.
1. b) The mechanical properties that WELDING ENGINEERS really appreciate.
Recently, developed low-hydrogen basic coated electrodes guarantee high mechanical properties in comparison with other non-alloy basic coated electrodes. Charpy V-notch impact values are more than 100 joules when tested at -30DC or – 46DC.
- c) Extra low-level of Diffusible hydrogen.
The flux coating of these electrodes has a unique feature, a barrier to moisture absorption and diffusible hydrogen to the weld metal. After 9 hrs of exposure at 27DC at 80% RH, the maximum moisture in the flux coating is 0.20% and diffusible hydrogen is less than 4ml!100g of the weld metal.
Fig 8 shows moisture absorption versus exposure time for recently developed E7018 class of electrodes.
Recent years have seen much emphasis on developing coated electrodes whose coating resist moisture pick-up. Such developments have permitted the user to test the electrodes for moisture pick-up with a view to extending the exposure time permitted by the structural welding code steel and other fabrication standards.
Moisture-resistant basic coated electrodes in accordance with AWS/SFA 5.1-91 or 5.1-2004 specifications are designated by adding the suffix letter “R” after the four digit classification number.
As per the specification, a letter “R” is a designator used with the Low-hydrogen electrode classification. The letter “R” is used to identify electrodes that have been exposed to a humid environment for a given length of time and tested for moisture absorption in addition to the standard moisture test required for classification of Low-hydrogen electrodes. As per the specification, “R” designated electrodes are exposed to an environment of 26.7°C (80°F) and 80% relative humidity for a period of not less than 9 hours by any suitable method. The moisture content of the electrode covering is then determined by any recommended method. The moisture content of the exposed covering should not exceed 0.40%, maximum specified moisture content for the electrode.
Diffusible hydrogen test
In addition to the absorbed moisture test, the basic coated low hydrogen electrode designated by an optimal supplemental diffusible hydrogen designator are tested according to one of the methods given in ANSI/AWS A4.3 Standard Methods for determination of the diffusible Hydrogen content of Martensitic, Bainitic and Ferritic steel weld metal.
For the purpose of certifying compliance with diffusible hydrogen requirements, the reference atmospheric condition shall be an absolute humidity of 10 grains of water vapor per pound (1.43g/kg) of dry air at the time of welding. The actual atmospheric condition is to be reported along with the average values for the test according to ANS/AWS A4.3-86 specification.
Two types of analytical apparatus are used for analysis of diffusible hydrogen.
- Mercury filled eudiometer
- GAS chromatography
The test specimen confined within its isolation chamber is held at the hydrogen evaluation temperature as per the following details.
Metallic mercury and vapours are hazardous and can be absorbed into the body by inhalation, ingestion or contact with the skin. Therefore all precautions involving the handling of mercury should be observed during measuring hydrogen by the mercury method.
On the, other hand, gas chromatography procedure is quite safe and is used by many European and Japanese manufacturers of basic coated Low-hydrogen electrodes. Since gas chromatography procedures vary from instrument to instrument, it is necessary to study carefully ‘the instruction and calibration procedures recommended by gas chromatography apparatus manufacturers.
The IIW mercury method may require 14 to 21 days for complete collection of all of the diffusible hydrogen in a test specimen at room temperature. The AWS method requires 72 hours at 45°C or 6hrs at 150°C for collection of nearly all of the diffusible hydrogen escape from the weld metal. Clearly elevated temperature speeds up the hydrogen escape from the weld deposit, but hydrogen does escape from solid steel at room temperature.
Effect of diffusible hydrogen content on tensile and ductility of the weld metal
Diffusible hydrogen in the weld metal could reduce tensile and ductility in a slow strain rate test.
Weld metal from a particular batch of E7018 electrodes which typically produces less than 5ml 100g of diffusible hydrogen averaged 27.3 % elongation in the as-welded condition and 28.5% in the “aged” condition.
However, differences in measured tensile, elongation between “as-welded” and “aged” conditions become much more important when electrodes of higher strength are used.
Resistance to moisture absorption characteristic of low-hydrogen basic coated electrodes
As mentioned earlier, basic coated low-hydrogen type electrodes have been developed having moisture-resistant coating. These electrodes have been developed on some of the following measures:
- Optimised granulametry of the coating flux ingredient.
- A newly developed binder system.
- Correct procedure used for removing almost entire quantity of moisture from the flux
These electrodes after baking have very low level of moisture in the flux coating. It is always less than 0.1 % and the typical value is as low as 0.04%. These electrodes when exposed to atmosphere having temp. 27°C +2°C and 80% RH absorb moisture very slowly. These electrodes are designated with the suffix “R” letter according to AWS/SFA 5.12004 specification.
Special packaging for low-hydrogen basic coated electrodes
It was shown by Franks in 1950 that Low hydrogen type coverings pick-up moisture when openly exposed to humid atmosphere and that different electrodes absorb different amounts of water under identical conditions. Smith studied the phenomenon in detail and demonstrated that different brands of electrodes have different tolerance to moisture absorption.
While manufacturing low hydrogen basic coated electrodes enough care is taken during baking operation so that almost all quantity of moisture is removed from the flux coating. Freshly baked electrodes can have as low as 0.04% moisture in the coating. Since flux coating of low hydrogen basic coated electrode is hygroscopic in nature, different manufacturers use different modes of packing.
- Low hydrogen basic coated electrodes are packed in polythene bags and then in cardboard cartons. These cardboard cartons are further shrink packed.
- The electrodes are hermetically packed into the metal tin immediately following production baking. These metal tins may have sufficient quantity of dessicant which can lower the relative humidity in the tin to below 8% ensuring that the electrode coating does not absorb moisture during storage.
- The electrodes are vacuum packed using special qualities of pouches and these vacuum packed electrodes are further packed in plastic or cardboard containers.
The metal tin containers used for vacuum packing are specially designed to provide protection for the electrodes during handling and storage.
Table 6 provide’ details of moisture level in the flux coating, shelf life of the electrodes for different packing conditions.
Saving in welding cost
Welding costs which varies from one fabricator to another depend upon the following factors.
- Drying of electrodes
- Storage of electrodes
- Preheating (Power, labour cost)
- Reduced labour productivity because working on
- preheated work place is more difficult
- Post weld heat treatment.
Recently developed Low hydrogen basic coated electrodes which are either hermetically packed or vacuum packed, deposit weld metal having highly reduced levels of diffusible hydrogen in the weld metal as well as in the heat affected zone. Such very low level of hydrogen increases tolerance to hardness in the heat affected zone and stresses applied to the weld metal without increasing the risk of cold cracking.
For many applications, where moderate preheat is usually required for eliminating the risk of cold cracking, recently developed vacuum PACKED BASIC COATED Low-hydrogen electrodes either eliminate preheating totally or reduce preheating temperature to a greater extent. Reducing preheating or eliminating preheating results in savings which is often equal to the cost of welding consumables.
Welding of A516 Gr 70 steel plate of thickness 40 mm. For X meter length of welding A516 Gr 70 steel thickness 40 mm.
|(a) Cost ofNormalE7018 electrodes||US $ 200
|(b) Cost of Vacuum packed E7018 electrodes
|(C) Cost of preheating maintaining inter
Pass temperature (1.5 times cost of electrodes)
|(d) Cost of storage, backing/drying of Electrodes
(30% cost of electrodes)
* * Fabricators pay US $ 50 more for vacuum packed electrodes whereas he saves US $ 360.
Hydrogen controlled consumables and welding consumable manufacturer’s capability and experience.
Manufacturers of basic coated electrodes have kept pace with the technological development taking place all over the world. They manufacture moisture-resistant electrodes of various classifications and these electrodes are now available in vacuum packed condition. Even though vacuum packed electrodes cost about 20 to 25% more than normal cardboard packed electrodes, these electrodes have a much longer shelf life and can be used straight on the job after opening the pack. However care should be taken so that the entire quantity of the packet is consumed within 4 to 8 hours after opening the pack. Usually vacuum packed low hydrogen elect odes are supplied in 2.0 kg pouches.
Recently developed basic coated low-hydrogen electrodes have improved toughness properties hence, these electrodes have a unique feature called resistance to ageing of the weld metal for manufacturing these electrodes, electrode manufacturers adopt the following measures:
- The use of low carbon clean wire.
- The use of extremely clean minerals, ferroalloys and oxides.
- The use of an optimized coating system with a high basicity index.
- Control of Non-Metallic oxide inclusions in the metal which controls the nucleation process during solidification of the liquid metal and favours the formation of a more fine grained structure.
It is expected that designers and consultants would demand increasing quantities of various, recently developed Low hydrogen basic coated electrodes which are either hermetically tin packed for eliminating cold cracking of the weld metal or HAZ of the parent metal.
- Hydrogen is an elusive element affecting cold cracking susceptibility in ferritic welds.
- Hydrogen control assumes criticality as the hardenability increases.
- There cannot be a compromise on hydrogen control.
- Procedural differences in measurement of hydrogen do require perfect understanding and careful comparative interpretations.
- Low-hydrogen basic coated electrodes, which are available in the market, deposit weld metal having low, very low or extra low level of hydrogen in the weld metal.
- Moisture resistant low-hydrogen basic coated electrodes are now available in the market.
- In order to have longer shelf life and cut down cost of welding, fabricators prefer vacuum packed basic coated Low-hydrogen electrodes.
- Recently developed low-hydrogen basic coated electrodes deposit weld metal having improved toughness properties and low level of hydrogen. These electrodes almost eliminate risk of cold cracking.