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| IMPROVE PRODUCTIVITY DURING WELDING OF STRAIGHT CHROME STEELS - PART 1: MARTENSITIC STAINLESS STEELS |
| INTRODUCTION |
| Stainless steels do not rust in air or corrode in contact with certain corrosive media. The ability of stainless steel to resist corrosion is due to high chromium content (minimum of 12%) in the steel. Chromium forms an oxide film on the metal surface which shields it from attack by atmosphere or corrosive substances. The shining surface is due to chromium in the steel. The chromium content varies between 11% and 27% (the minimum is 12% and it goes even up to 32% in 312 grades); the corrosion resistance varies with the corrodant, structure of the SS and the chemistry of the SS; the 18% Cr stainless steel is resistant to general corrosion while the higher percentage Cr stainless steel is ferritic and is resistant to stress corrosion. Stainless steels which do not contain nickel or contain it in insignificant quantities (below 2%) are called straight chrome steels.There are many groups of stainless steels like austenitic, ferritic, martensitic, duplex, super duplex etc. The ferritic and martensitic grades contain only chromium and are commonly referred to as straight chrome steels, as mentioned above. The different groups of stainless steels differ in their composition, properties and corrosion resistance. So they are to be used in specific environments for which they are suitable. They also differ in their weldability and call for suitably designed procedures to produce sound welds that will perform well in service. In this article we will discuss how to obtain defect free weld when welding one grade of straight chrome steels - the martensitic stainless steels. |
| WHAT ARE MARTENSITIC STAINLESS STEELS? |
| Martensitic stainless steels are stainless steels containing primarily about 12Cr and 0.2C. Actually there is a specific relation between C and Cr and that has to be maintained. Addition of other alloying elements may require changes in the percentages of C and Cr to obtain a martensitic structure. They solidify as martensite under normal cooling and exhibit martensitic structure at room temperature. |
| HOW DO THEY DIFFER FROM AUSTENITIC GRADES? |
The austenitic grades like the 300 series are the most popular stainless steels. The martensitic grades which are in the 400 series are less popular and they differ from austenitic grades in many ways.
- These steels are harden able unlike austenitic grades
- Poor weldability, ductility and toughness
- These grades are magnetic unlike austenitic grades
- Thermal conductivity is higher than austenitic grades but thermal expansion is lesser
- Good general corrosion resistance
- Harder than austenitic grades and hence better wear resistance, erosion resistance
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WHAT ARE THE POPULAR GRADES AND THEIR
COMPOSITIONS? |
| The martensitic grades fall in the 400 series of compositions. Some of the popular base material grades and their typical compositions are as given below. The martensitic grades find usage in applications where general corrosion resistance together with wear resistance is required like hydro power plant turbine blades, pumps, valves, etc. |
| GRADE | Typical composition |
| 410 | 0.13C-12Cr |
| 420 | 0.16C-13Cr |
| CA6NM | 0.06C-13Cr-4Ni-0.5Mo |
| CA15 | 0.14C-12Cr-1Ni-0.5Mo |
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| HOW ABOUT THEIR WELDING BEHAVIOR? |
| As indicated earlier, the martensitic grades harden during cooling because of martensite formation. So in welding the martensitic stainless steels, number of precautions are required similar to welding of carbon and low alloy steel; these includes preheating, slow cooling and post weld heat treatment. The austenitic grades of stainless steels do not require preheat and post weld heat treatment but the martensitic grades require preheating and post weld heat treatment. The table given below will give general guidelines for preheat and post weld heat treatment which is normally based on the carbon content of the stainless steel. |
| CARBON CONTENT | PREHEAT DEG C | PWHT |
| <0.05 | 120 | OPTIONAL |
| 0.05-0.15 | 204 | RECOMMENDED |
| >0.15 | 315 | REQUIRED |
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The ASME recommends a min preheat of 204°C for P6 group, which pertains to straight chrome steels. Just as in the case of welding carbon and low alloy steels, because of martensite formation, the martensitic stainless steels are susceptible to hydrogen induced cracking and use of low hydrogen electrodes is recommended. So in effect, to obtain defect free welds and avoid rework,
- Use preheat, high inter pass temperature, slow cooling, post weld heat treatment
- Use low hydrogen electrodes
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| SELECTION OF CONSUMABLES |
| The martensitic grades of stainless steels can be welded either with matching composition electrodes or with austenitic grade electrodes. The table below lists the advantages and disadvantages of using the 2 types of consumables. |
MATCHING COMPOSITION
CONSUMABLES | AUSTENITIC TYPE
CONSUMABLE |
| Hardenable and hence preheat, PWHT required | Non harden able and hence no preheat, PWHT |
| Matching corrosion properties | Corrosion properties not matching |
| Good wear properties | Wear properties not as good |
| Not suitable for dissimilar steels welding | Suitable for dissimilar steels welding |
| Weld deposit susceptible to cracking | Weld deposit generally not susceptible to cracking |
| Economical | Will be costlier than matching composition electrodes |
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| ADOR WELDING LTD. ELECTRODES |
| We have in our range both matching and austenitic type of electrodes for welding martensitic grades of stainless steels. They are as given below. |
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| The addition of alloying elements such as Ni, Mo helps in making the stainless steel tougher and it also improves the weldability. Our electrode Betachrome 13/4 deposits weld metal with enhanced impact properties even after heat treatments. Please click on the name of respective electrode to know more about its applications and characteristics. |
| CONCLUSION |
Martensitic stainless steels find applications in typical industries because of their unique properties. Welding of these steel is therefore a necessity and maintaining the desired properties is also a challenge. This can be achieved by understanding the material behavior and following the carefully designed welding procedures.
In the next issue of eWeldone, we will discuss in detail another grade of straight chrome steels: the ferritic stainless steels.
Please contact us at cmo@adorians.com for more details on this subject and for help in selecting the right consumables for these applications. |
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