Chapter 7 Welding Metallurgy of Stainless Steels

Question 1

1. A certain family of stainless steels can suffer from changes in grain size when heated and thus a reduction in toughness. Which one of the following is this?

a. Ferritic stainless where grain growth reduces toughness
b. Martensitic where the freezing of the component to change grain size reduces toughness
c. Ferritic stainless steels where grain size decreases with heat and there is a subsequent loss of toughness
d. Precipitation hardening steels where quenching has a drastic effect on grain size and toughness

Answer 1

a. Ferritic stainless where grain growth reduces toughness

Question 2

2. What elements can be added to a precipitation hardening (PH) stainless steel in order to produce the small precipitates necessary for hardening?

a. Carbon and iron
b. Aluminum, copper and niobium
c. Hydrogen and nitrogen
d. Chromium, nickel and molybdenum

Answer 2

b. Aluminum, copper and niobium

Question 3

3. Which of the following groups of alloying elements drive the formation of the austenite phase in stainless steels?

a. Chromium, silicon and manganese.
b. Carbon, chromium and molybdenum
c. Nickel, manganese and nitrogen.
d. Nickel, chromium and manganese.

Answer 3

c. Nickel, manganese and nitrogen.

Question 4

4. When using the gas metal arc welding processes to weld stainless steels, small percentages of carbon dioxide in the shielding gas can lead to:

a. carbon pickup in the weld deposit.
b. more uniform penetration.
c. smoother weld bead appearance.
d. higher melting rates.

Answer 4

a. carbon pickup in the weld deposit.

Question 5

5. When welding austenitic stainless steels which method is typically recommended to avoid solidification cracking in the weld metal?

a. Austenitic consumables that produce a small, but controlled quantity of delta ferrite in the solidified weld metal.
b. Filler materials that deposit less than 0.03’ carbon.
c. Stabilized ferritic consumables.
d. High heat inputs with weaving

Answer 5

a. Austenitic consumables that produce a small, but controlled quantity of delta ferrite in the solidified weld metal.

Question 6

6. What are the main factors that can cause the onset of stress corrosion cracking (SSC) in austenitic stainless steels?

a. A tensile stress and the presence of chlorine or other halogen ions.
b. Niobium carbides and tensile stress.
c. A quenched and tempered structure
d. A sensitized structure under bending loads

Answer 6

a. A tensile stress and the presence of chlorine or other halogen ions.

Question 7

7. Which element in a stainless-steel alloy is mostly responsible for the resistance of the material to high-temperature oxidation?

a. Nickel
b. Molybdenum
c. Copper
d. Chromium

Answer 7

d. Chromium

Question 8

8. Creep strength is the ability of materials to resist distortion during long-term exposure to loads at high temperatures. Which of the following stainless steel has the best “creep strength”?

a. The austenitic 304H grade
b. The austenitic 316L grade
c. The 2205 duplex stainless steel
d. The 410-martensitic grade

Answer 8

a. The austenitic 304H grade

Question 9

9. Which two regions of a PH stainless steel weld zone can exhibit a reduction in strength after completion of welding?

a. Weld metal and base metal side of the fusion boundary.
b. Base metal side of the fusion boundary (weld interface) and the overaged region in the HAZ where precipitates coarsen
c. Weld metal side of the fusion boundary and the region that reached a temperature of approximately 1040°C.
d. The entire HAZ and base metal fusion face.

Answer 9

b. Base metal side of the fusion boundary (weld interface) and the overaged region in the HAZ where precipitates coarsen

Question 10

10. The higher electrical resistivity of austenitic stainless steels can result in what advantage over carbon steels when using the GMAW process?

a. Higher wire feed speeds leading to better deposition rates
b. Reduced UV radiation and thus less tendency to form ozone in the local environment
c. Less wear on consumable tips in the GMAW gun
d. Will increase the cooling rates and thus allow better HAZ properties with PH steels

Answer 10

a. Higher wire feed speeds leading to better deposition rates

Question 11

11. There is a phase in certain stainless steels that can cause serious embrittlement. What is this phase and how does it form?

a. The martensitic phase which is over tempered thus causing a hardening of an already hard structure.
b. In a duplex stainless, an imbalance of phases after welding resulting in more of the brittle ferrite phase at low temperatures.
c. Overheating of the precipitate phase in a precipitation-hardened steel will cause serious embrittlement
d. The formation of sigma phase after prolonged heating can cause embrittlement

Answer 11

d. The formation of sigma phase after prolonged heating can cause embrittlement

Question 12

12. When comparing a duplex stainless steel to a ferritic stainless steel, in what areas would the duplex have improved overall properties?

a. They have better resistance to heat tinting during preheating
b. The duplex materials do not undergo a phase change as do ferritic stainless steels
c. The duplex stainless steels have a single phase but this phase is resistant to grain growth, unlike the ferritics.
d. They have superior toughness and corrosion resistance

Answer 12

d. They have superior toughness and corrosion resistance

Question 13

13. Austenitic stainless steels can be stabilized by adding amounts of certain elements to the steel chemistry, what are these elements?

a. Large amounts of strong austenitic forming elements have to be added.
b. Small amounts of manganese and molybdenum need to be added
c. Small amounts of solution treatment elements are added.
d. Add small amounts of strong carbide formers

Answer 13

d. Add small amounts of strong carbide formers

Question 14

14. When selecting a welding consumable for joining martensitic stainless steel, what type of consumable would be best if matching strength and hardness is not a factor?

a. An electrode with matching composition
b. An austenitic stainless-steel electrode
c. A martensitic stainless-steel electrode.
d. A high nickel-based electrode.

Answer 14

b. An austenitic stainless-steel electrode

Question 15

15. In the selection of stainless steels, which type would be preferable for use in applications needing good corrosion resistance together with wear resistance?

a. The BBC ferritic type of stainless.
b. The martensitic type with relatively high carbon content.
c. The preferred solution would be to overlay a carbon steel with the higher carbon duplex type of stainless.
d. The FCC, precipitation hardened type in the overaged condition.

Answer 15

b. The martensitic type with relatively high carbon content.

Question 16

16. What property does an FCC austenitic stainless steel have that will make the size of the weld
    pool larger for the same heat input when compared with welding a BCC carbon steel?

a. The FCC lattice dissolves more carbon thus making it resistant too electrical input from the welding arc.
b. The melting point of the FCC stainless is much higher than the BCC carbon steel
c. The welding arc is much smoother and more stable with an FCC consumable, thus making it easier to penetrate the base material under the welding arc.
d. The melting temperature and thermal conductivity of the FCC austenitic stainless is less than that of the BCC carbon steel.

Answer 16

d. The melting temperature and thermal conductivity of the FCC austenitic stainless is less than that of the BCC carbon steel.

Question 17

17. When austenitic stainless steels are used in equipment that will operate in the range 600-750 deg C, what type of stainless should be selected?

a. The stabilized 321 or 347 types.
b. The high carbon types designated as HC
c. The low carbon L type with carbon less than .03%
d. The type that is made austenitic by the use of nitrogen and manganese rather than nicked

Answer 17

a. The stabilized 321 or 347 types.

Question 18

18. Stress corrosion cracking is a particular problem with specific stainless steels, stress situations and environment. Which of the following is the most susceptible?

a. Ferritic stainless welded at 200 deg C and then exposed to an oxygen-rich atmosphere
b. Austenitic stainless steels in the as-welded condition with an environment that may contain chlorides.
c. Duplex stainless steels, stress relieved after welding and operating in a general corrosion environment.
d. Post weld stress relieved austenitic stainless Steels in a halide environment.

Answer 18

b. Austenitic stainless steels in the as-welded condition with an environment that may contain chlorides.

Question 19

19. In order to reduce the probability of solidification cracking in FCC austenitic stainless steels there are a number of approaches. Which of the following is correct?

a. Arrange for welding consumables to always solidify as austenite first and preheat to a minimum of 150 deg C.
b. Arrange for the weld zone to be put in compression and use carbon steel filler that solidifies as BCC ferrite first.
c. Arrange for welding consumables to contain a small amount of delta ferrite or use very low sulphur materials.
d. U5e austenitic plate that is < than 6% nickel and use stabilized 316L consumables.

Answer 19

c. Arrange for welding consumables to contain a small amount of delta ferrite or use very low sulphur materials.

Question 20

20. There are several compositional diagrams available for stainless steels such as the WRC and Schaeffler diagram. What is the purpose of such diagrams?

a. They offer the welding engineer a unique way to calculate heat inputs
b. They help calculate preheats and from this the estimated cooling rates and composition of the HAZ
C. They basically direct welding procedures by dictating preheats in order to establish final weld metal compositions.
d. To estimate the final composition of the weld metal knowing the items to be welded and the approximate dilution of the welding procedure to be used.

Answer 20

d. To estimate the final composition of the weld metal knowing the items to be welded and the approximate dilution of the welding procedure to be used.

Question 21

21. Martensitic stainless steels are very hard which is why they find use in situations that require wear and erosion properties. What drives the hardness of these stainless steels?

a. A unique annealing heat treatment.
b. The combination of nickel and molybdenum
c. The relatively high carbon content
d. The hardenability properties of the unique FCC lattice that martensitic steels enjoy

Answer 21

c. The relatively high carbon content

Question 22

22. With 8% nickel added to the basic iron/carbon structure, how much of another element is added to make the alloy stainless and austenitic at room temperature?

a. The element chromium must be added to 18%
b. The element carbon is added up to 0.08%
c. Only 10% chromium is needed to make the alloy austenitic
d. Up to 3% molybdenum is required to be added to the basic 8% alloy in order to make it fully austenitic.

Answer 22

a. The element chromium must be added to 18%

Question 23

23. When joining ferritic stainless steels with matching electrodes, the welding procedure must be carefully controlled to avoid the following:

a. the entrapment of small interstitials such as oxygen and nitrogen which can cause severe embrittlement
b. the entrapment of large interstitials such as hydrogen which can become trapped in the FCC lattice and cause embrittlement
C. the entrapment of slag when using the FCAW process, which can cause severe corrosion in certain specific environments
d. the pick-up of carbon from the GTAW process using carbon dioxide shielding which can increase the carbon content of the L grade ferritic stainless steels.

Answer 23

a. the entrapment of small interstitials such as oxygen and nitrogen which can cause severe embrittlement

Question 24

24. In a duplex stainless steel, what elements can be added to the electrode/shielding gases to assist in the development of the austenitic phase?

a. Hydrogen can be added via the shielding gas
b. Increased nickel and nitrogen via the electrode and/or the gas shield
c. Carbon and oxygen in the electrode and from the carbon dioxide shielding gas
d. Chromium additions to the electrode or filler wire only.

Answer 24

b. Increased nickel and nitrogen via the electrode and/or the gas shield

Question 25

25. It can be an advantage to use argon or helium shielding gases with GTAW and GMAW welding process when welding the austenitic stabilized grades (321 and 347). Why is this?

a. Reactive elements such as titanium and aluminum can be transferred across the arc without significant loses.
b. Reactive elements such as chromium and nickel can be transferred across the arc without significant loses.
c. Since there is no slag formed by these processes, there is no carbon transfer into the slag.
d. Reactive elements such as titanium and aluminum are not transferred across the arc.

Answer 25

a. Reactive elements such as titanium and aluminum can be transferred across the arc without significant loses.

Question 26

26. Why is it acceptable to use the gas shielded flux cored welding process on stainless steels with shielding gases that are relatively high in carbon dioxide?

a. The slag formed with the process protects the weld from excessive chromium pickup
b. The slag formed with the process protects the weld from carbon pick up.
c. The carbon dioxide in the shielding gas forces the transfer into spray and therefore produces a shallow, less diluting penetration.
d. It’s acceptable only with the low carbon grades of austenitic stainless steels as a welding procedural method for gaining some strength without subsequent heat treatment.

Answer 26

b. The slag formed with the process protects the weld from carbon pick up.

Question 27

27. When joining a carbon steel plate (ferritic BCC) to an austenitic stainless steel (FCC), what is the accepted practice?

a. Complete the joint using filler materials that match the chemistry of the carbon steel plate.
b. Complete the joint, using a high dilution welding process, with matching austenitic electrodes
c. Complete the joint, using a low dilution process, with duplex stainless electrodes which give a balance of FCC and BCC in the final mix.
d. Butter the carbon steel side with a 309-type filler, using a welding process that gives acceptable dilution, and then complete the joint with conventional stainless steel electrodes.

Answer 27

d. Butter the carbon steel side with a 309-type filler, using a welding process that gives acceptable dilution, and then complete the joint with conventional stainless steel electrodes.

Question 28

28. Once pitting corrosion is initiated what is the relationship between the pit and its growth rate in the stainless steel in question?

a. In the majority of cases the growth rate of the pit increases as the depth of the pit increases
b. In most cases the pit will heal after an elapsed period of time, depending on the environment.
c. The depth of the pit is of no concern as the growth is stopped by the formation of new chromium oxide on the pit walls
d. The pit will not grow in environments that include chlorine and will self-heal in such conditions.

Answer 28

a. In the majority of cases the growth rate of the pit increases as the depth of the pit increases

Question 29

29. If martensitic stainless steels are joined using FCC austenitic electrodes, certain precautions may still need to be taken. Which of the following would be a correct approach7

a. Use the prescribed preheat so as to reduce the probability of cracking in the HAZ.
b. Use a high heat input to increase the HAZ grain size and thus maintain toughness to prevent HICC
c. As hot cracking is a probability then select electrodes that will deposit elements that will reduce this probability
d. Upon completion of welding, immediately quench with water spay to reduce the probability of solidification cracking.

Answer 29

. a. Use the prescribed preheat so as to reduce the probability of cracking in the HAZ.

Question 30

30. The heat treatment of martensitic grades depends on the level of temperature and time. According to the isothermal transformation diagram, if we cool the material to 700 BC and maintain that temperature for one day, what kinds of microstructure can be expected?

a. Martensite
b. Ferrite
c. Austenite, ferrite and carbide
d. Austenite all the way

Answer 30

c. Austenite, ferrite and carbide