In this page we explain some of the technical termiology in the maketfield of the daily business of BBS-Systems. We hope, to give you a small overview of the special terminology you also can use with your activities.

Objective
This procedure has been prepared to provide training personnel with the understanding of fundamentals as related to Quality Assurance/Quality Control within the Quality Assurance Management, Inc. Organization.

Scope
This procedure will demonstrate the importance Quality Assurance /Quality Control as it apply to the inspection of high purity piping systems in accordance with specifications, codes, and written standards.

Terms and Definitions
General

Audit
The evaluation of a system or procedure to verify it`s effectiveness.

Calibration
The standardization of an instrument to a known reference standard.

Certification
Written testimony of qualification.

Documentation
Evidence in verification of facts, proof (or writing) to support a test or inspection.

Guideline
A suggested practice that is not mandatory.

Inspection
The performance of examination, testing, measuring, verifying, witnessing or reviewing to determine conformance to specified requirements.

Inspector
Personnel certified to verify conformance to specific requirements.

Owner
The person, group, agency or corporation who has title to the facility.

Procedure
A written document that specifies how an activity is to be performed.

Qualification
The abilities gained through education, training or experience as measured against established requirements (i.e. test, standards).

Qualified

Procedure
An approved procedure that has demonstrated the ability to meet specified requirements.

Requirement
A mandatory practice intended to comply with a standard.

Shall
Term used to indicate that a provision is mandatory.

Should
Term used to indicate that a provision in NOT mandatory.

Specification
A precise statement of a set of requirements to be satisfied by a material, product, system, or service.

Training
To impart the knowledge and skills necessary to perform a particular function.
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Material Inspection

Source Inspection
Material inspection performed at vendor`s or manufacturer`s location.

Receiving Inspection
Material inspection performed at the job site.

Installation
Inspection Material inspection performed during installation.

Material Test

Reports (MTR`s) These reports are generated by the steel mill that produces the stainless steel and show the chemical make-up of the steel as well as many of the physical properties of the steel. An analysis is made for each batch of the steel. This could cover as much as 100 tons of steel. Some of this steel is made into tubes and some is made into other materials. All of the batch will have the same heat number. The (MTR) or Material Test Report will contain all of this information.

Heat Number See MRT above.

ASTM
American Society of Testing and Materials.

Wall Thickness
Thickness of tube or fitting, normally measured in thousandths of an inch.

Min. Wall
Thickness This tubing`s minimal allowable wall thickness will be the nominal wall thickness.

Ovality
The measurement of the "out of roundness" of a tube. It should take the difference between the largest diameter measured and the smallest diameter measured on a tube.

Nominal
Refers to the designed wall thickness, NOT the actual wall thickness.

Seamed Tubing
Tubing made from a flat sheet that is rolled into a cylinder and welded.

Seamless Tubing
Tube made by forcing a hollow over a mandrel, creating the desired wall thickness and outside diameter.

Rockwell Hardness
This is a test performed to measure the actual hardness of the steel. Typically, the Rockwell Hardness desired level is < 90. Compression fitting manufacturers like the value to be < 80 so that it will be easier for the ferrules to "bite" into the steel. The E. P. people like the hardness to be > 90. This seems to produce a smaller granular structure in the stainless that provides a smoother surface for their process

Surface Anomaly
Any occurrence on the surface of a tube, fitting or machined surface that is unexpected or differs from the remainder of the surface or expected surface conditions. These would include occlusions, inclusions, pits, stringers and machining marks.

Occlusions
Voids in the metal.

Inclusions
Impurities on and into the surface. Metal carbides are a typical Problem.

Annealed
This is a process to stress relieve Tubing after it is redrawn at the mill. The temperature they use for this stress relief is the major factor in granular size at the surface. Normally, the annealing process takes place in a hydrogen environment. The temperatures used are in the 1600 to 1800° F range.

Vacuum Arc Remelt (VAR)
This is method used to remove some of the volatile impurities in the stainless steel like sulfur.

Bar Stock
This is a solid "bar" of steel. "Machined" valve bodies are machined from this bar. One 20`length of bar may make as many as 60 to 80 valves.

Squareness
This is a measurement of the perpendicularly of the end of a tube to the run of the tube or the branch of a tee to the run of the tee. It is measured in degrees or millimeters.
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Electropolishing

Electropolishing
Process that involves the chromium enrichment of the surface in stainless steel materials. This process uses mild acids (usually phosphoric, sulfuric and surfactants) with an electrode to facilitate the removal of metals from the surface of the material

Passivity
The preferential oxidation of a surface performed by an oxidizing acid like nitric acid. This is usually a combination of nitric, sulfuric and phosphoric acids in Electropolishing solutions.

Out-guessing
Trace non-condensable contaminants emission from a surface.

Free Metal
Non-oxidized metal.

Oxidized Metal
Surface metal that is combined with oxygen to form an oxide.

Intergranular Corrosion
(Etching) When a too aggressive Electropolishing procedure is performed on the surface of a tube, the area between the grains can be corroded. This will cause a non-preferential corrosion which leads to additional corrosion. Typically, corrosion products can be observed on SEM micrographs and identified with EDX or Auger spectroscopy.

Carbide
Precipitation Carbide precipitation can occur in the intergranular area of the stainless steel. When it occurs here, it is the result of carbon combining with chrome. Normally, you will first have some corrosion occurring between the grains before carbide precipitation occurs.
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Welding Field Terminology

Automatic

Orbital Butt Weld
A weld made by a machine without the addition of filler metal.

Weld Bead Wander
Trace elements or impurities in the weld can cause the weld bead to wander from side to side. If severe wandering occurs, the weld bead can slip off to one side enough to completely miss the butted tubes.

Weld Discoloration
This refers to discoloration or oxidation in the heat affected zone of the weld. The heat-affected zone is an area about 1 to 2 millimeters to both sides of the weld. Normally, the color referred to is a light blue or yellow. Often, it takes a proper light and angle to distinguish the color. Apparently, the color is cased by light refraction and is not an actual "lue" or "yellow".

Purge Gas
Refers to Argon and is used to prevent oxidation at the weld side.

Shield Gas
This purge gas protects the tungsten from oxidation.

Back-up Gas
This purge is on the I.D. side of the weld and protects the I.D. from severe or excessive oxidation.

Convexity
As viewed from the outside of the tube, this would be a raised area of the weld.

Concavity
As viewed from the outside of the tube, this would be viewed as a depression of the weld.

Penetration
This refers to the complete melt of the steel at the butted sight during the welding process.

Downslope
Once the weld has been completed, the current being applied must be gradually reduced or a "pi" hole may be formed in the weld. This gradual reduction is called a "downslope". It takes the weld current from the last needed for welding to the 0 level in a few seconds.

Weld Program
The setting on an automatic orbital weld machine.

T.I.G.
Tungsten inert gas, used in arc welding.
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Test

Surface

Roughness
The unevenness of a flat surface. This is usually expressed in values of Ra, Rmax or Rz. The units are in micro inches.

Profilometer
This is a tool that measures the roughness of a surface. It uses a stylus that is dragged over the surface. The up and down movement is recorded as roughness. The profilometers have a limit as to their LDL because of the physical size of the stylus. The following are (2) types of Profilometers: Surphometer and Perthometer.

SEM
Scanning Electron Microscope.

ESCA
Electron Spectroscopy for Chemical Analysis.

Auger (AES)
Auger Electron Spectroscopy.

Micro Inch
One millionth of an inch.

Ra
Surface roughness measuring the average value for the mean to peak height.

RMS
The "route mean square" of the roughness average.

Rmax
The maximum roughness measurement within the stroke of the measuring tool.

Rz

STM
Scanning tunneling microscopy.

T.O.C.
Total organic carbon, used to qualify the cleanliness levels of D.I. water systems. This measurement is an indication of the amount of micro biological activity in a UPW system.

Resistivity
The resistance of a material to allow a flow of electricity, used to describe the inorganic concentration in UPW systems. The maximum resistivity attainable for water is 18.5 million ohms (mega-ohms). An salts added to water will decrease its resistivity. Just a few ppb of sodium will degrade the resistivity to below 10 mega-ohm.
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Influence of low ferrite content on the corrosion behavior of stainless steels

Dr. R. Morach, Head of Materials Technology, Ciba Specialty Chemicals Ltd., Basel (Switzerland) P. Ginter, Welding Technology, Novartis Services Ltd., Basel

Today processes in chemical industry demand superior construction materials. Statistical analyses show chemical attack and manufacturing defects are the most common source of failure. Most often observed, however, is localized corrosion. General corrosion attack is easily detected in laboratory tests and will probably not occur in day to day operation of the plants. Among the localized corrosion phenomena crevice corrosion and pitting corrosion are related to process parameters and may be controlled through operating procedures. Selective corrosion, stress corrosion cracking and any other corrosion attack near or of the weld, respectively, are caused by manufacturing procedures and are not very easily controlled.

In consequence, the Basle Chemical Industry specifies superior quality materials and manufacturing procedures for its chemical equipment. The following measures show how an optimum corrosion resistance of chemical equipment made from stainless steels can be achieved. Especially the influence of delta-ferrite on corrosion behavior is discussed.

Stainless steels and delta(–)ferrite (1,2)

A large number of different alloys are stainless steels. In common they have a minimum chromium content of 12%. Chromium and nickel, the other important alloying element, behave quite different: while nickel favours the formation of an austenitic lattice, chromium enhances the formation of the ferritic crystal. Other alloying elements show a behaviour either similar to chromium or nickel, respectively:

Ferrite forming elements:
Chromium, molybdenum, titanium, niobium, tungsten, vanadium, silicium, aluminum

Austenite forming elements:
Nickel, carbon, copper, nitrogen, cobalt, manganese

Depending on the content of alloying elements the stainless steels show different structures:
martensitic Cr-Steels
Ferritic Cr-steels
Ferritic-austenitic Cr-Ni-(Mo-N)-steels (duplex steels)
Austenitic Cr-Ni-(Mo-N)-steels with or without ferrite.

Studies of A. Schäffler and W. Delong and others revealed the well known Schäffler-Delong-diagram to estimate the structure of an alloy or a weld. To understand the process of solidification the knowledge of the phase diagrams is essential. From the ternary phase diagram or iron-chromium-nickel (fig. 1) it is obvious that depending on composition the primary solidification either starts with primary –ferrite formation followed by delta-transformation or with primary austenite formation. Melts near the border of primary ferrite/primary austenite tend or retain ferrite in the austenitic structure due to kinetic hindering of the delta-transformation. This retained ferrite content is controlled by cooling rates or by welding parameters e.g. heat input, gas composition....

Other structural defects are well known in stainless steels:

Phosphorous and sulfur having a low solubility in austenite tend to form low melting-point phases at the grain boundaries, which cause microfissuring and hotcracking. This segregation can be controlled by a higher ferrite content, because ferrite has a good solubility for phosphorous and sulfur.

Generally, impurities collect in the residual melt, thus forming segregations in the final solidification zone.

Furthermore the formation of intermetallic phases is observed: Carbides (M23C6 and M6C), sigma phase (FeCr), Chi phase (Fe36Cr12Mo10) and Laves phase (Fe2Mo), whose formation is controlled by the solubility of carbon and nitrogen. Nitrogen is added in high molybdenum steels to retain a nonsegregated structure.

Measurement of –ferrite content (3,4)
Different methods are available to determine the content of –ferrite:
Nonmagnetic methods magnetic methods
Metallographic section saturation magnetism
Phase determination by X-ray magnetic force
Electrical resistance measurements magnetic permeability
Estimate from Schäffler-Delong

The methods above are based on different measuring principles, thus each method yields a different ferrite content. The estimate according to Schäffler-Delong is only a rough estimate. From metallographic sections only a two-dimensional view of a three-dimensional distribution is available. With the magnetism based methods every magnetic phase in the structure contributes to the result.

Measuring the magnetic permeability is commonly used, because the method is suitable to examine large areas. There are still two principal problems:
Primary standards
Determining the absolute-ferrite content of a structure is not possible. Therefore primary calibration standard have been created and made available to every days use through secondary standards.

Magnetic phases

Measuring the permeability accounts for every magnetic phase present in the structure (e.g. deformation martensite). The permeability of the ferrite phase itself depends on the content of alloying elements; i.e. a highly alloyed ferrite tends to show a weaker magnetic reaction as the same amount of a low alloyed ferrite.

To overcome these limitations the ferrite content of a structure is expressed in ferrite numbers (FN). From primary standards (soft iron blocks having a nonmagnetic layer atop) magne gauges are calibrated for the calibration of the secondary standards. Using the secondary standards portable devices (ferritoscope) may be used in the work shops and on site.

Basler Norm BN2 (Basle standard 2) (11)

In the early sixties the chemical industry around Basle came to the conclusion, that stainless steel structures with multiple phases or segregations are less corrosion resistant. Segregations and intermetallic phases lower the resistance against localized corrosion (pitting, intergranular attack, selective corrosion). The selective corrosion of the ferrite phase and the increased corrosion attack of stainless steels due to an increased ferrite content of the alloy were observed in lab experiments. Therefore the Basle Chemical Industry defined a standard stainless steel, which based on the steel DIN 1.4435 with a restricted chemical analysis:

Table 1: Composition of DIN 1.4435 and 1.4435 according Basle Standard 2 (BN2)

Additionally the nitrogen content was restricted to less than 0.1%, thus minimizing the precipitation tendency. The restricted analysis made it possible to improve the quality, i.e. the band-width of corrosion performance of the steel was narrowed.

With the influence on the corrosion resistance in mind the ferrite content of this steel was limited, too:
<0.2% in the base material
<0.5% in the weld.

This limitation in ferrite content is the very special feature of the Basle Standard 2. By limiting the ferrite content, the Basle Chemical Industry forced at that time the steel making companies to reduce the phosphorous and sulfur content to avoid microfissuring an hotcracking. The reduced contents were also beneficial to the corrosion resistance of the selected steels.

Measurements of the ferrite content showed, that it may be estimated from Schäffler-Delong, if a steel meets the requirements of Basle Standard 2:
Chromium equivalent – 0.91 (Nickel equivalent) <7.70
With Chromium equivalent = %Cr + 1.5(%Si) + %Mo + 2(%Ti)
Nickel equivalent = %Ni + 0.5(%Mn) + 30(%C) + 30(%N – 0.02)

If the above formula yields less than 7.7, the measured ferrite content drops safely below 0.5%.

The specification of a maximum ferrite content had a simple quality control benefit: The low ferrite values can be reached using over-alloyed welding consumables and applying a strict control of welding parameters. Thus, through the measurement of the ferrite content the quality in the manufacturing process could be controlled.

Practical realisation of low ferrite content (5,11)

Weld joints having less than 0.5% ferrite content require some special attention during manufacturing. It was shown by different experiments (Tab. 2) that the low ferrite content according to Basle Standard 2 can be achieved by:

Table 2: Delta ferrite content of stainless steel tube welds (Closed head) (böhler, eig)

Overalloyed welding consumables
Beside special gas mixtures in orbital welding, filler material in the form of rings and made from Inconel 600 (Fig. 2) may be placed between the tubes to achieve the low –ferrite content, the high amount of nickel guarantees a stable austenitic weld joint.

Consumables of DIN 1.4519 or DIN 1.4439 are used for manually welded joints or for orbital welding with open head. This stainless steel is overalloyed in reference to nickel and molybdenum. Again a fully austenitic weld joint is achieved. For critical applications the use of the heavily overalloyed DIN 1.4453 may be indicated.

Shielding gas with nitrogen
The beneficial effect of nitrogen on the austenitic structure and the corrosion resistance has been discussed thoroughly. When adding nitrogen to the shielding and the backup gas, the weld joint takes up the nitrogen and thus the ferrite content is lowered. The best performance was achieved using Argon with 2% Nitrogen added. These special mixtures are used for orbital welding processes.

The application of the different possibilities at the Basle Chemical Industry is summarized in the following figure 3.

Corrosion resistance (5-11)
The corrosion resistance of duplex structures has raised some discussions. Generally it has been accepted, that different compositions in the different grain structures favour the formation of microscopic electrochemical corrosion cells and selective corrosion is observed. In consequence, the less resistant phase is dissolved, while the more noble phase is electrochemically protected. Additionally, the phase boundaries lower the corrosion resistance, too. The detrimental effect of residual phases in a given structure has been examined in depth. –phase, deformation martensite and various precipitations reduce the corrosion resistance.

Even low –ferrite contents (0-5%) reduce the corrosion resistance of stainless steels (fig. 4 and 5). Above 0.5% d-ferrite content the corrosion resistance of an alloy is definitely reduced compared to the ferrite free material. The observed corrosion attack proceeds along the ferrite lines, the mechanism is somewhat similar to inter crystalline corrosion.

Our own experiments and literature reviews confirm, that either strongly oxidizing or reducing conditions may promote the selective corrosion of –ferrite. Sheets of 4mm thickness were welded with heavily overalloyed welding consumables for additional tests in cooperation with the Swiss Federal Institute of Technology. The measured –ferrite content and the –ferrite distribution is given in the following table.

Structure from metallographic sections --ferrite content

Base material Austenitic structure with lines of –ferrite 0.2%
Heat affected zone Austenitic structure, --ferrite close to the weld 0.4%
Weld Austenitic structure, some segregation, no –ferrite <0.1%

The metallographic section shows, that the center of the weld is free of –ferrite, only a small seam of –ferrite is found at the border to the base material.

In immersion tests with 30% sulfuric acid and 3% NaC1 the –ferrite phase was attacked first. From electrochemical tests in aqueous 5% NaC1 solutions two areas of corrosion attack were observed. Along the border weld/heat affected zone pits were formed. These pits could be correlated to the –ferrite structure. The pits in the center of the weld were most probably caused by segregations.

In potentiostatic polarization experiments the pitting incubation time (time to current increase beyond a limit) is measured. The shortest incubation times were observed in the heat affected zone (fig. 6a) near the weld. These findings correspond well to the pit initiation sites found in the FeC13 test (fig. 6b).

In further experiments the micro-electrochemical methods (fig. 7) developed at the Swiss Federal Institute of Technology were used. First an etched sample was covered with a grid of indents made with a microhardness tester. Afterwards the sample was polished, the hardness indents being still visible. The microelectrochemical probe with a capillary diameter of less than 20 µm was positioned between the indents and the local current density potential curves were measured. These localized experiments showed, that the pitting potential in the base material and the weld were more noble than in the heat affected zone. The variation of the pitting potential in the heat affected zone was larger than in the base material and the weld. This can be explained from the very localized measurement area of the microprobe. In some experiments segregation or –ferrite grains present in the exposed surface lowered the pitting potential, while in other measurements the homogenous material without defects was tested.

Again, from metallographic section it was observed, that the attack begins on the –ferrite structure.

Only little information is available on the influence of the –ferrite content on the corrosion behaviour of stainless steels in high purity water. With the results above in mind, also in biological and pharmaceutical application a low –ferrite content of the stainless steels used has to be achieved. Surface defects from localized attacks on the –ferrite structure may deteriorate the sterile conditions of such systems.However, there are some other, more pronounced effects that may or may not be correlated to the –ferrite of the steels:

Rouging

Sometimes tubes and other equipment made from stainless steels show in high purity water at ambient temperatures a red or yellow colored surface. Beside the –ferrite content additional parameters like sterilization procedures, ozone, UV light and surface quality may contribute. For a better understanding the influence of each parameter on the "rouging" effect should be investigated.

Corrosion attack

Some tubes in ultra purity water systems (<0.1µS) were found covered with a red/brown powder, which was analyzed to be a corrosion product of the stainless steels used. The ferrite content varied along the length of the tubing, but never exceeded the value specified in the Basle Standard 2.

Again, --ferrite could cause the problem, but it must be considered, too, that ultra pure water is very corrosive.

However, --ferrite statements should not be over-estimated. Analyzing the corrosion problems observed in our specialty chemicals, the pharmaceutical chemical production and agribusiness chemical production, there was not one single case in the past ten years, where the –ferrite content caused the failure of the equipment. Most often chemical attack was too severe for the steels used, Other failures were related to manufacturing, e.g. welding with weld consumables identical to the base material, welding defects and residual stresses in the heat affected zone. All these problems were observed on stainless steels according to Basle Standard 2.