2_niro standard 1.61

7/27/2019 2_Niro Standard 1.61

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Standard 1.61

1998-10-01 Stainless Steel - General Requirements Page 1/6

Niro A/SNiro A/S Gladsaxevej 305 PO Box 45 DK-2860 Soeborg Denmark Tel +45 3954 5454 Fax +45 3954 5800 Reg No 12682

Contents Page

1. Preface.........................................................................2

2. Fabrication premises and aids.................................2-3

3 Materials.....................................................................3

4. Placement of welds..................................................3-4

5. Cutting and forming...................................................4

6. Welding....................................................................4-5

7. Surface treatment....................................................5-6

8. Inspection and testing ................................................6

9. Finished equipment....................................................6

Stainless SteelGeneral Requirements


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1. Preface

1.1 Scope

This standard is applicable to all Nirostainless steel equipment.

Failure to comply with this standard mayresult in severe damages, especially relatingto corrosion. Corrosion resistance ofstainless steel in general is briefly describedin the following.

1.2 Corrosion resistance

The reason for the good corrosion resistanceof stainless steel is the presence of a verythin - but chemically and mechanically

strong - film of chrome oxide on thesurface.

This corrosion resistance depends in a veryhigh degree on the quality of the surfacefinish. The more corrosive a media is, thegreater the demands will be concerning thequality of the surface finish.

As a main rule a cold-rolled, pickled at millssurface will have almost optimum corrosionresistance. It is therefore extremely

important to maintain the original surfaceduring the whole fabrication process.

1.3 Carbon content

At a temperature range between approx.500C and 850C, chrome carbides aregradually formed resulting in the reductionof corrosion resistance of stainless steel.

The amount of separated chrome carbidesdepends on the exact temperature, the time

in which the steel is in the criticaltemperature range, i.e. the total heat inputand cooling time, and most of all thecontents of carbon in the steel.

The formation of chrome carbides leavesthe welded zone with reduced chromecontent, thus reducing the corrosionresistance. The zone becomes sensitized.

When sheets of thickness less than approx. 5mm are welded, the heat input willnormally be small, and the corrosionresistance will not be essentially decreased.

However attention must be paid to cross-welds, repair-welds etc., where the totalheat input may be extra large.

It is recommended to use stainless steel witha maximum carbon content of 0.03% (L-

quality) or a titanium/niob stabilized steel, ifparts in large thickness have to be weldedor, if the design includes details which aredifficult to handle for the welder.

1.4 Stainless steel quality

The selection of the specific stainless steelquality is among others based on theaggressiveness of the corrosive media.

In this connection it is important to

emphasise that the corrosion resistance ofthe complete component seldom is as goodas for the selected stainless steel type. Thereason for this may be a non-optimumsurface finish or a presence of small cracksand pores, where a heavy corrosiveenvironment may be generated.

Crevices and other geometric details, whichmay give possibilities for local heavycorrosive environment, shall be avoided inthe design. If this is not possible, the use of

a more resistant stainless steel for thosedetails are to be considered.

Surface finish requirements and acceptablegeometrical defects shall be considered inrelation to the actual corrosive environmentand selected material quality.

2. Fabrication premises and aids

2.1 Storage

All stainless steel material shall be stored ina suitable manner in order to avoid damageor contamination from chlorides andweather etc. It shall be stored in areasseparated from carbon steel materials.Insulation materials shall be stored in aclean and dry area.

2.2 Fabrication

Stainless steel fabrication shall be carriedout in dedicated areas. These areas shall in

the greatest possible extent be separatedfrom areas, where carbon steel fabrication isbeing carried out.


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The supplier shall take all practical measuresto avoid contamination of the stainlesssteel. In order to avoid any damage of theoriginal surface, plastic foil covering shouldbe considered.

2.3 Tools

Hand tools of carbon steel material shall notbe used. Hand tools which were used withcarbon steel shall be absolutely avoided.Care has to be taken while using the liftingequipment.

Rolls, forming equipment and shearingblades used to form or cut stainless steelshall be kept clean and burr-free. Any debrisfrom carbon steel forming or cutting are to

be removed prior to use of such equipmentfor stainless steel.

Hammers, earthing lugs or clamps etc. usedon stainless steel should preferably be madeof stainless steel or a material compatiblewith stainless steel.

Grinding and polishing wheels, discs or beltsshall be dedicated for use on stainless steel.

3. Materials

Only materials conforming to the specifiedrequirements may be used. All materialsshall be inspected by the supplier prior touse, i.e. check of steel quality, dimension,surface finish etc.

3.1 Surface quality

Exact values for surface roughness forstandard plates/sheets (e.g. 2B) are notavailable, as they vary from mill to mill for

the different thickness of sheet/plate.

In case the surface quality is better on oneside of a plate than on the other, the betterside shall be used for product contactsurface.

3.2 Traceability

If traceability is required, the supplier shallmaintain a system of identification for thematerials used in manufacture. The suppliershall supply a material list stating drawingand item number for the relevant parts withreference to the material certificates.

3.3 Material identification markings

No markings with hard stamping on plateswith 6 mm thickness or less are allowed,unless this is specifically required.

In any case, the hard stamped marking shallnot be on the product contact surfaces.

Material identification markings should notbe left on stainless steel surfaces subject tocorrosive environment, unless this isspecifically required.

4. Placement of welds

4.1 General

The placement of welds shall be carefullyconsidered prior to plate cutting. Thenumber and extent of welds shall be kept toa minimum, in order to obtain a uniformappearance of the finished equipment.

Where practical possible, the equipmentshall be made from a single sheet/plate.Small pieces of plates shall be avoided.

Accumulation of welds shall be avoided.Distance between two parallel butt welds

shall at least be the greater of 50 mm or 10times the plate thickness.

Intersecting welds shall be avoided to thegreatest extent possible. Longitudinalseams, which adjoin a circumferential seamfrom each side shall be separated by at least50 mm.

Corner welds in drains, ducts, vessels andvessel outlets shall not be used unlessspecifically approved by Niro.

Openings for manways, pipe connectionsetc. as well as reinforcement rings shall beplaced as far from the shell joints aspossible, and shall not cut into other joints.

Fillet welds securing external attachmentsshould be placed at a minimum toe to toedistance of 10 mm away from any existingseam welds.


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4.2 Pneumatic hammers

The position of the impact point shall beproperly selected in order to avoid crackingin the parent material.

Distance from impact point to any weldshall be at least 400 mm.

Provided the weld is tested by X-rayexamination, the distance from the impactpoint to a high quality square butt weld canbe 100 mm. The cost for the X-rayexamination shall be borne by the supplier,unless otherwise agreed upon.

5. Cutting and forming

The material may be formed to the requiredshape by any process that will not impairthe quality and the proportions of thematerial for its intended use.

All burrs and ragged edges shall beremoved prior to welding. Any cut edgethat is left in the cut condition shall bedressed to remove sharp edges.

Cold forming or bending of the materialshould be carried out by gradually applied

pressure. Hammering of the material as aforming method should be absolutelyavoided.

The material thickness shall not be reducedby more than 10% of the original thicknessduring any general forming operation. Fordomed/dished ends, where a greaterdistortion is unavoidable, the minimumdesign thickness shall be maintained.

Knuckle radii shall be free from kinks,

ripples or dents. Minor forming marks onthe non-contact side are acceptable.

Deviations in the diameter of bendpipes/tubes shall not exceed 10% of theoriginal diameter.

6. Welding

Welding personnel shall be adequatelytrained. The welding process, especially theheat input, shall be carefully controlled and

appropriate welding methods, gas mixturesand filler materials shall be used.

6.1 Joint preparation

All burrs and ragged edges shall beremoved prior to any welding operation.

Joint preparations made by machining,

grinding, laser or water jet cutting areusually acceptable. If other methods areused, e.g. arc gouging/cutting, the fusionfaces shall be ground clean to removeoxides.

Prior to welding, the fusion faces and theadjacent material, for a minimum of 25mmfrom the fusion faces, shall be free frommoisture, scale, paint, grease, plastic foil orother foreign matter.

6.2 Assembling and alignment

Weld joints shall be aligned and held inposition by methods not damaging thematerial.

Any tack weld that is to be incorporated inthe final weld shall conform with thecurrent welding quality requirements. Thetack weld shall be made using a weldingconsumable of the same composition as ofthe final weld.

Tack welds shall be positioned in such a waythat they will be incorporated fully into themain welded joint or can be removed afterwelding has been completed.

Temporary attachments used to assist fit upshall be compatible with the parentmaterial and shall be removed withoutdamaging the equipment. After removal oftemporary attachments, the remnants ofthe welds shall be ground flush, polished

and passivated.

All permanent attachments to equipmentshould conform with the shape of thesurface to which they are attached.

6.3 Gas protection

When welding the austenitic stainless steel,it shall be done carefully and aimed toobtain the same corrosion resistance as theuntreated parent material.

For this reason the root of the weld shall beprotected from oxidation/contamination byback purging with a suitable gas.


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Acceptable limit for oxygen contentdepends on the stainless steel type and thetype of protection gas used.

After welding the heat affected zone on theroot side shall have a yellowbrown or

lighter colour. The heat affected zone onthe welding side shall have an even colourand must not be black.

If the colour in the heat affected zone isdarker than specified above, pickling shallbe made without any extra costs to Niro,even if such pickling is not specified.

For pipe welds etc., where surfacetreatment on the root side is not possible, itis very important to ensure adequate gas

protection during welding.

6.4 Filler material

Filler materials shall be compatible with thematerials being welded. The quality of theweld metal shall at least correspond to thequality of the parent material.

Recommendations from recognizedsuppliers of filler materials shall beobtained.

Different filler materials shall be properlyidentified and stored.

6.5 Welding

In general the welding quality of theequipment shall be in accordance with NiroStandard 1.4-, unless otherwise specified.

Every effort shall be made to avoid stray arcstrikes on the material surface. In case ofstray arc strikes, these shall be dressed,

polished and passivated.

The equipment will be rejected in caseplastic foil covering is burned into theparent material.

The use of permanent backing bars or ringsis not permitted but temporary backing barsor rings may be used.

Reinforcements and other attachments,which are attached by intermediate fillet

welds, shall be fixed by welding in theends/corners of the reinforcements.

Where accessible, all welds and heataffected zones shall be cleaned prior tofurther surface treatment. i.e. all slag, fluxresidues, spatter etc. shall be removed.

7. Surface treatment

7.1 Grinding and polishing

During grinding and polishing localisedheat build-up and blueing of the materialshall be avoided.

Grinding and cutting dust from stainlesssteel, which is burned into the surface, willbehave as ordinary carbon steelcontamination. The chrome in the dust willdisappear during heating, and the particles

will, therefore, no longer be corrosion-resistant.

Reduction of material thickness duringgrinding and polishing shall not exceed 5%of the original material thickness. Localreduction of up to 10% of the platethickness may be accepted

7.2 Chemical surface treatment

A chemical surface treatment is used for

optimizing the surface corrosion resistance.

Carbon steel contamination may normallybe removed by decontamination. It mustnot be expected that mechanical cleaningalone will remove the carbon steelcontamination effectively as in many casesthe carbon steel particles will be pressedinto the stainless steel surface.

Prior to any chemical surface treatment, thesurface shall be thoroughly cleaned and

degreased. Cutting lubricants, rests of glueand other grease deposits on the surfaceshall be removed.

Passivation chemicals shall not be left onthe material surface for a period exceedingthat recommended by the manufacturerand all residues shall be completelyremoved using fresh, clean water.


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8. Inspection and testing

The supplier shall carry out adequateinspection during fabrication. Niro reservesthe right at any time to check by means ofthe most appropriate test methods toascertain that the equipment conforms tothe specified requirements.

Non-destructive testing shall be carried outin accordance with Niro Standard 1.5-, whenrequired.

All welds in light gauge plate on tanks,vessels, bins, etc., which are designed tocontain liquid, shall be leak tested by thesupplier, e.g. by penetrant, pneumatic leaktesting or water filling to overflow.

Only components which are designed tocontain liquid may be filled with water.

9. Finished equipment

Finished equipment shall be properlycleaned.

Plastic foil on internal surfaces of closed

vessels, tanks etc. shall be removed. Plasticfoil on external surfaces may remain on thecomponent, except in cases where theequipment will be transported or stored indirect sunlight and/or extreme heat.

The equipment shall preferably be storedindoor, but large constructions may bestored outdoor provided that they areproperly protected.

2_niro standard 1.61

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