Colorado School of Mines Department of Metallurgical and Materials Engineering MTGN Metallurgy of Welding Prof. Stephen Liu

Transcription

1 Colorado School of Mines Department of Metallurgical and Materials Engineering MTGN Metallurgy of Welding Prof. Stephen Liu Exam #3 Posted: December 3, 2001 Due: December 10, 2001 (Answer only 8 out of the 9 questions!) Name: 1. Weld Chemical Composition and Dilution a.) Define delta quantity and the terms included in the delta quantity equation. (20%) b.) For a base metal dilution of 65%, calculate the delta quantity for the elements C, Cr, and Ni in the following stainless steel weld metal. (40%) (wt.pct.) C Cr Ni Mn Si Mo Base Metal Filler Metal Analytical c.) Estimate the microstructure of the stainless steel weld using the calculated ( theoretical ) values. Explain your procedure and tools used for the estimate. (15%) d.) Discuss the different phase regions regarding their location on the Schaeffer Diagram and their relationship with alloy composition and weld cooling rate. (25%)

2 2. Weld Solidification and Cracking Behavior a.) The macrophotograph below shows a crack in the underbead region (Svensson 94). Hydrogen cracking is known to occur in the coarse grain heat affected zone (CGHAZ) of steel weldments. Discuss why this is a preferred location for hydrogen cracking. (50%) b.) The SEM photograph below shows a solidified weld metal (Svensson 94). Identify the solidification substructure(s) and the weld defects that you observe. Speculate as to why the defects occurred and how would you mitigate the problem. (50%)

3 3. Metal Transfer Modes in Arc Welding The following questions concern the Metal Transfer Mode of a welding process: a.) If a welding process exhibits the voltage characteristic as shown in the figure below, draw the current vs. time behavior for this process. (25%) b.) How does the slope of the voltage vs. time graph vary with inductance? (25%) c.) What do the minimum values of the voltage vs. time graph represent? (20%) d.) Draw the voltage vs. time graphs for the metal transfer modes listed below. (Include a set of voltage criteria for MTM definition.) (30%)

4 4. Laser Beam welding Process Efficiency Beam-Material Interaction a) Despite the many excellent characteristics of Laser beam welding, it is considered a low efficiency process. Select two main factors that you believe are responsible for the low process efficiency and discuss them. (50%) b) Why are some metals less suitable for Laser beam welding? Name two examples of metals that you would expect poor LB weldability. Explain your speculation and the consequences of using this process for such materials. How would you resolve the difficulties to obtain quality LB weldments from these metals? (50%)

5 5. Thermal Experience in Weldments a.) The temperature distribution in spot welding is typically modeled using a stationary heat source. Express, mathematically, the functional form of this distribution. What are the assumptions and/or limitations in using this solution to model the spot welding process? (30%) b.) Given the following temperature-time diagrams for a SMAW and SAW weldment, compute the cooling rate at T = 1000 C for both processes. Which weldment has the greater cooling rate? How can the difference in cooling rates between the two processes be explained in terms of the net heat-input into the weldment? (40%) c.) Using the same temperature-time distribution as in Part (b) above, what is the value of t 8/5 for both the SMAW and SAW process? How might the difference in values affect the microstructure of the low carbon, structural steel weldment? (30%)

6 6. Arc Welding - Power Sources - Porosity a) Semi-automatic GMAW typically uses a CV power source. Describe the major characteristics of this type of power source. Discuss "self-adjustment" and explain how this feature is used in welding process control. (30%) b) Ar, Ar-1H 2, Ar-6H 2, He, He-1H 2, He-6H 2, and N 2 were used as shielding gas in magnesium welding in Lab #2. Discuss the effect of shielding gas on weld pool protection for metals with great reactivity such as magnesium. (35%) c) Still in connection with the shielding gases in part (b), discuss the effect of hydrogen on porosity formation. Relate your discussion with hydrogen solubility in liquid and solid magnesium. Include schematic hydrogen solubility vs. temperature diagram as part of your discussion. (35%)

7 7. SMAW Consumables and Steel Weldability a.) Select two major types of SMA electrodes for steel welding and compare their characteristics regarding welding performance, shielding mechanism, flux composition and melt characteristics, weld metal mechanical properties, and potential industrial applications. (30%) b.) Using the E6010 and E7018 grade electrodes as examples, discuss the importance of electrode baking and the criticality of selecting an appropriate baking temperature. Assuming that you had established a baking temperature of 450 o C for both electrodes, what would you expect from the two electrodes regarding welding performance and bead morphology. (30%) a.) You are to join two steels plates of a pressure vessel with AISI designation The composition in wt. % is 0.40 C, 0.88 Mn, 0.95 Cr, and 0.20 Mo. Discuss the weldability of this material and what special processing, if any, is needed. (40%)

8 8. Arc Physics a.) Define all terms in the Saha Equation as given below. (25%) α p evi = 1 α κt 2 5 C T 2 exp 2 po b.) Discuss the influence of α, T, and Vi on welding arc stability. Use the graph below to illustrate and support your discussion. (75%) 20.0 Graph plotted using data from Lab #2 (F2001) 17.5 Arc Voltage (V) Helium Argon Arc Current (A)

9 9. Weld Metal Phase Transformations Given the following two CCT diagram for steel weldments (Zhang & Farrar), b.) Determine the IIW carbon equivalent for each of the two above alloys and comment on their respective weldability. (30%) c.) Using the right-hand-side diagram, discuss the effect of cooling rate on weld metal microstructure. (35%) d.) Using both diagrams, discuss the effect of weld chemical composition on the resulting microstructures when cooled at C/s. (35%)