Fracture Toughness

Papers to look up
1. Bhadeshia HKDH, DebRoy T. Sci Technol Weld Joining 2009;14:193–6. (Santos 2010)
2. Bott IS, de Souza LFG, Teixiera JCG, Rios PR. Metal Mater Trans A 2005;36:443–54. (Santos 2010)
3. Bangaru NV, Fairchid DP, Macia ML, Koo JY, Ozekcin A, et al. Proceedings of the pipeline tech. Ostend 2004:789–804.
4. Bonnevie E, Ferrière G, Ikhlef A, Kaplan D, Orain JM. Mater Sci Engng A 2004;385:352–8.
5. Ikawa H, Oshige H, Tanoue T. Trans Jpn Weld Soc 1980;11:87–96.
6. http://www.sciencedirect.com/science/article/pii/S026130691400870X
7. https://www.onepetro.org/conference-paper/ISOPE-I-11-104
8. Microstructure and mechanical properties of friction stir welded 18Cr–2Mo ferritic stainless steel thick plate (looked up and downloaded)
9. Lan et al (looked up and downloaded)
10. Kumar et al (made interlibrary loan)
11. https://books.google.com/books?hl=en&lr=&id=3i5yIJ_p-48C&oi=fnd&pg=PA371&dq=cooling+rate+FSW+fracture+toughness&ots=qLge1xKohE&sig=90qam9rVQUslrthzveCIl83bxb8#v=onepage&q=cooling%20rate%20FSW%20fracture%20toughness&f=false
What is a MA (mertensite-austenite constituent)?
Authors used so far
1. Santos (2010)
2. Xiao long (2015)
3. Tribe
4. Avila 2015
5. Kulekci 2012
6. Aydin 2013
7. Lakshminarayanan
8. Fujii 2013
9. Cota 2000
10. Scott Rose
11. Lan 2010 - not completed
12. Lan 2013 - not completed
13. Kumar 2010
14. Nandan 2008
15. Allred 2013
16. Wei 2009
What are delaminations, crack front tunneling, and dimpled rupture (Avila 2015)
HI
1. low HI has not been explored (tribe)
2. High levels have bad FT (tribe)
3. HI has Strong effect on as-welded microstructure (Aydin 2013)
4. Lower the HI produces smaller PAG and finer grain/lath structure resulting in higher strength and lower ductility (Aydin 2013)
5. The heat generated during the welding process is equivalent to the power input introduced into the weld by the tool minus some losses due to microstructural effects (Fujii, 2013)
6. Subtopic 6
7. Heat input is the best process variable to correlate with post-weld microstructures since it captures the effect of the specific energy of the FSW process (Wei 2009)
8. The peak temperatures in FSW of steels are estimated to be in the range of around 1000 to 1200 C, or about 70 to 80% of the melting temperatures (Rose 2013)
9. Wei found that the heat input had the best correlation to hte weld microstructure (Rose 2013)
10. Heat input controls cooling rate in the weld, an increase in heat input results in slower cooling rates (Wei 2009)
Tensile Strength
1. travel speed has greater effect than spindle speed (Lakshminarayanan,2013)
2. Relationship between TS and HI. TS goes down as HI goes up (Aydin 2013)
  1. Subtopic 1
3. Relationship between TS and hardness. Strength goes up as hardness goes up (Aydin 2013)
  1. Subtopic 1
4. Tensile and yield strengths increase when the cooling rate increases or when the finish-cooling temperature decreases (Cota 2000)
5. The tendency of the yield strength to increase with increase of the cooling rate is a consequence of the interruption of hte accelerated cooling at temperatures greater than or equal to 400C, which prevented the formation of martensite (Cota 2000)
Defects
1. Potentially caused by low HI coupled with insufficient z force (tribe)
2. There is an empirical control parameter to perform sound FSW welds used in .... lab in Brazil: the width of the bead must constant and equal to the diameter of the shoulder durin the weld. THis procedure guarantees absence of defects and was established based on athe necessity of the shoulder to hold the material underneath it. This procedure was used in the present study (Avila 2015)
Variation in CTOD
1. most likely weld inhomogeneity (tribe)
2. CTOD values in the advancing side are 50% of those in the retreating (tribe)
3. As the notch position varies the crack samples a highly inhomogeneous microstructure in the SZ, which leads to variation in FT (tribe) See Horschel and Fairchild et al
X80
1. Composition (Aydin 2013)
2. In HSLA steels, the precipitation of fine microalloying carbides and carbonitrides are not the predominant factor for increasing strength. THe predominant factor is fine bainitic microstructure. (Aydin 2013).
3. which have additions of titanium, niobium and vanadium for grain refinement and carbide precipitation hardening (Santos 2010)
4. Composition (Santos 2010)
  1. Subtopic 1
Sample Processing
1. Etching
  1. 2 % Nital for 20 seconds (Aydin 2013)
  2. 4 vol.-% nitric acid +96 vol.-% ethanol solution (Fujii 2013)
  3. To reveal the morphology of MA constituent, the polished specimens were etched with LePera reagent (Lan 2010)
2. Polishing
  1. through 1 um (Aydin 2013)
3. Hardness testing
  1. LECO LM 100AT tester diamond pyramid indenter with 500g load, 15s dwell, 400 um indentaion (Aydin 2013)
BM X80 mainly composed of elongated fine-grained polygonal ferrite (Aydin 2013)
Microstructure
1. higher HI produced swirl-like mircostructure (Aydin 2013)
2. This bainitic microstructure consists of thin, relative straight and long, parallel ferrite laths with discontinuous carbide particles (second phases) at the lath boundaries. This microstructure indic ates that the HZ region has reached a peak temperature in excess of A3, which is the temperature at which ferrite transforms to austenite on heating, and the cooling rate is sufficient to form lath bainite (Aydin 2013)
3. The WN microstructures are predominantly coarse bainitic structure with carbines along the lath boundaries (Aydin 2013)
4. Microstructure images
  1. BM microstructure (Aydin 2013)
  2. HZ microstructure (Aydin 2013)
  3. HAZ microstructure (Aydin 2013)
  4. WN microstructure (Aydin 2013)
5. Blocky M/A constituents are generally beneficial to the improvement of toughness compared with M/A strip (Aydin, 2013)
6. bainitic lath structure becomes thinner with increasing cooling rates (Aydin 2013)
7. Peak hardness values in the HZ regions of hte samples decrease almost linearly with increasing HI, bainite lath width, and PAG size. (Tribe?)
8. It is well-known that the strength/hardness of the macroscopic material is proportional to the reciprocal of the square root of grain size, according to the Hall-Petch relation (Aydin 2013)
9. FSW samples with lower HI had higher strength as a result of finer lath microstructures resulting from higher cooling rates (Aydin 2013)
10. The commercial ISO 3183 X80M steels, which are equiv- alent to the American Petroleum Institute (API) 5L X-80 steels have typical microstructures consisting of ferrite and bainite. In order to increase strength without significant losses in toughness, a high volume fraction of bainite is desirable. (Santos 2010)
11. The MA constituent morphology plays an important role in the fracture toughness, with elongated and massive MA resulting in lower and higher fracture toughness, respectively. (Santos 2010)
12. This microstructure presents moderate amount of MA constituent with massive morphology in the ferritic matrix, called granular bainite. According to Bangaru this microstructure is beneficial to fracture toughness. Bott et al have reported that massive MA constituent may result in elevated toughness in these high strength steels, which has been corroborated by the elevated CTOD-values measured in this work. (Santos 2010)
13. The stir zone microstructure has been more severaly influenced by the welding process and parameters (than MA constituent?) Santos 2010
14. MA has an elongated morphology for low cooling rates and its morphology changes to massive as the cooling rate increases. The microstructure observed in this work, have shown that the MA morphology have changed from slightly acicular for the 0.2 mm/revolution joint to massive for the 0.33 and 0.29 mm/revolution joints. . . the MA morphology evolution observed in this work does not agree with the results reported by Ikawa. could be due to the deformation effect unexpected heat generation rate changes or to the important chemical composition differences between teh studied alloys. The large Mn content difference should be having a very important effect on the MA formation and therfore, on its morphology (Santos 2010 citing Ikawa)
15. The stirred zone in API steels share similar characteristics with the coarse grain heat affected zone (CGHAZ) in arc welds, according to Fairchild et al, this could explain in part the low toughness in some welds, however, the mentioned authors did nto report the downwards forces and it is difficult to generalize (Avilia 2015)
16. Coarse bainite structure in SZ (Avila 2015)
17. The M-A remains as one of the biggest sources of low CTOD values in the HAZ of arc welds, while in FSW the wak regions have a tendency to be the SZ and the SZ(HZ), even though, the FSW joints also presented considerable presence of M-A within the joint. (Avila 2015)
18. There were CTOD values belonging to the SZ and HAZ from joing 3, which presented low toughness; however the HAZ-1 notch was not affected, which suggests that the second pass relieves the stresses in pass 1 for this condition, as indicated by the hardness map in Fig .2 (Avila 2015)
19. The refinement of the bainitic ferrite by the colling rate could be due to the following: when the cooling rate increases, the reduction in the Bs temperature leads to an increase in the drivin force (the difference of the free energy between austenite phase and ferrite phase) for the nucleation rate of sub-units of ferrite and, consequently, to a decrease of the width of bainitic ferrite laths. Increase in the cooling rate leads to a continuous increase of the Vickers hardness (Cota 2000)
20. Boron increases bainitic hardenability (Cota 2000)
21. High misorientation packet boundary in coarse bainite seems to have few contributions to the improvement of the toughness because cleavage fracture micromechanism of coarse bainite is mainly controlled by crack initiation (Lan 2010)
22. the influence of MA constituents and of crystallographic bainite packets on cleavage fracture micromechanisms is closely related with testing temperature (Lan 2010)
23. It is evident that the carbon atoms segregate on the M-A constituent and carbon concentration on the slender M-A constituent is higher than that on the masive M-A constituent (Lan 2012)
24. Lan (2011) supported this idea by concluding that lath martensite of high cooling reate specimen has higher density of high misorientation grain and packet boundaries than coarse bainite of low cooling rate specimen according to EBSD results. High misorientation boundaries in lath martiensite play an important role in imporiving the crack propagation energy because they can deviate and/or arrest microcrack propagation effectively.
Spindle Speed
Tools
1. PCBN-WRe (Avila 2015)
Welding Transient distance
1. 25 mm (Avila 2015)
Crack energy
1. Can be separated into crack initiation energy and crack propagation energy. THerefore the variations in MA constituent and matrix crystallographic feature can be connected with the changes of crack initiation energy and crack propagation energy respectively. (Lan 2010)
Input Parameters for PCBN
1. Spindle speed - 300 rpm, travel speed, 51mm/min - performed fracture toughness at -20C (Kumar 2010)
2. Tribe inputs
  1. Subtopic 1
3. Relationshiop between Torque and travel speed (Nandan 2008)
  1. Subtopic 1
4. Relationship between rotational speed and peak temperature (Nandan 2008)
  1. Subtopic 1
HZ
1. Increase hardness by lowering HI (Tribe)
  1. increase due to formation of martensite at lower HI (Tribe)
2. SZ higher hardness at advancing side, high concentrations of lath martensite and upper bainite (Tribe)
3. Cooling rate is the dominate factor affecting hard zone formation through its effects on weld microstructure (Rose)
4. Decrease in heat input the bainitic structure in the hard zone becomes finer and so hard zone strength increases (Aydin 2013)
5. Located on the top surface and in the weld nugget (WN) (Aydin 2013)
6. HZ region experienced high strain and/or strain rate whith aided in the transformation to austenite (Aydin 2013)
7. HZ may exhibit lower toughness than the WN region as it has more M/A strips as reported by Tribe (Aydin 2013)
8. Hardness of weld zone as a function of HI (Aydin 2013)
  1. Subtopic 1
9. The decrease in peak hardness in the HZ and WN are the result of coarsening laths and PAG structure with increasing HI and/or slower cooling rate (Aydin, 2013)
10. Both the expansion and decreasing peak hardness of the HZ region with increasing HI provides evidence of strong dependence of the HZ microstructure and properties on weld HI (Aydin 2013)
11. The HZ region exhibits the highest yield and ultimate tensile strength, but this is accompanied by the lowest elongation: 43% lower than the BM and 23% lower than the WN (Aydin 2013)
12. hardness of the liquid CO2 cooled joint is higher than that of the naturally cooled joint (Fujii 2013)
13. Hard zone in advancing side, peak temperatures, see figure 1 (Avila 2015)
14. Cooling rate vs HZ vs PAG size (Lan 2010)
  1. Subtopic 1
15. The highest hardness regions in the stir zones were typically coincident with regions that displayed the coarsest prior austentite grain size and the highest volume fractions of martensite and/or bainite. It is believed that the peak hardnesses in these friction stir welds are likely influenced by the large grain size contribution to hardenability (Kumar 2010)
16. Nelson et al also observed that the hard zone occurred in welds of high peak temperature, as a result of high heat input. Grain refinement, due to fast cooling rates, has also been suggested to be a couse of the hard zone (Rose 2013)
17. At constant backin plate thermal conductivity, the hardness increases with decreasing heat input. (Rose 2013)
18. The increase in hardness with decreasing heat input is contrary ot the findings of Nelson et al who aboserved the hard zone increase in hardness with increasing heat input (limited to single backing plate) Rose (2013)
19. The backing plate also has a significatn effect on the hardness. At a constant heat input, the hardness increases iwth increasign backin plate thermal conductivity. Becase heat can leave the weld faster at higher thermal conductivies, this significance cooling rate may be influencing the hardness (Rose 2013)
20. Hardness as a function of heat input (Rose 2013)
  1. Subtopic 1
21. This microstructure indicates that the HZ region has reached a peak temperature in excess of A3, which is the tempearture at which ferrite transforms to austenite on heating, and the cooling rate is sufficient to form lath bainite (Aydin 2013) In addition, Wei et al reported that the HZ region experienced high strain and/or strain rate which aided in the transformation to austenite. (Aydin 2013)
22. Cooling rate is the dominate factor affecting hard zone formation through its effects on weld microstructure (Rose 2013)
23. The HZ usually accounts for a small fraction of the overall area in the weld; however hardness avlues in the HZ can exceed the SZ by as much as 30%
24. Jominy tests with cooling rates greater than 136 C/s exhibited lath ferrite that closely resemble lath structures found in the weld HZ (Allred 2013)
25. The AS side has a higher peak temperature than the RS, but it is most likely no large enough to make a difference. It is most likely due to higher cooling rates on the AS. These are caused by hot extruded material from the RS being deposited on the cooler AS. (Allred 2013)
26. The HZ of FSW X65 predominantly consists of lath ferrite which results in a decrease in toughness. Though FSW is a highly plastic deformation process, lath ferrite in the HZ is primarily a result of accelerated cooling rates and peak temperature (Allred 2013)
CTOD
1. Other authors have reported low high CTOD at room temperature (Santos). However Fairchild reported low fracture toughness in welds made with a W-Re tool and high welding speed (250 and 180 mm/min) (Avila 2015)
2. Santos performed two FSW passes in 12 mm thick plates of API X80 and reported that the downward force changed for each condition and was higher for the higher spindle speed where they reported the lowest CTOD values. (Avila 2015)
3. low CTOD = low FT (Avila 2015)
4. Furthermore Horschel found that the same trend in two FSW passes i nplates of API x65. (Avila 2015)
5. The minimum CTOD values for the base metal are over 0.4 mm at all temperatures tested. The CTOD data scatter due to delaminations in the BM has been related to the presence of material banding, hot rolling texture (Avila 2015)
6. Authors such as Kumar et al and Fairchild et al considered CTOD values between 0.1 - 0.25 mm as a safe interval for application offshore and onshore. (Avila 2015)
Increase in FT
1. HAZ toughness increased (tribe)
  1. lower bainite at low HI (tribe)
2. HSLA 65 FT increase with RPM (tribe)
3. X80 - increase with decreased spindle speed (tribe)
  1. no appreciable difference between SZ and HAZ FT (tribe)
4. travel speed has greater effect than spindle speed (tribe,Lakshminarayanan 2013)
5. Highest toughness results from low spindle speed and low HI (tribe)
6. Fracture toughness increased linearly with decreases in both HI and spindle speed (Tribe)
  1. Optimal parameters for FT in X80 (tribe)
7. Additionally, there is no evidence of grain coarsening in the HAZ. GIven the lack of grain coarsening and slight tempering observed in the FSW HAZ; it is likely that the HAZ in the FSW will exhibit much greater toughness than that observed in arc weldments of the same steel (Aydin 2013)
8. Dependent on specimen geometry, microstructure, phase composition, and temperature (Kulekci 2012)
9. It is clear that the fracture toughness of FSW lap joints increases exponentially as the hardness reduces (Kulekci 2012)
10. Joints produced with lower spindle speeds presented higher toughness at the heat-affected zone (HAZ) and stir zone (SZ), which are compared with the base metal (BM) toughness. (Santos 2010)
11. The joints produced with low spindle speeds showed CTOD values above the offshore standard requirements (Santos 2010)
12. This steel is manufactured in order to form some MA< which is desirable to increase the fracture toughness (Santos 2010)
13. The CTOD-values measured at the SZ and the HAZ of samples welded with 0.33 and 0.29 mm/revolution parameters, which correspond to 300 and 350 rpm spindle speeds, respectively, indicate excellent fracture toughness (Santos 2010)
14. The 0.20 mm/revolution joint resulted in degenerated upper bainite associated with the elongated MA and high carbide (cementite) content which was detrimental to fracture toughness. On the other hand, the 0.29 and 0.33 mm/revolution joints presented massive MA associated with acicular ferrite (granular bainite) and lower carbide content which was beneficial to fracture toughness (Santos 2010)
15. Downward force and spindle speed are the parameters that presented the strongest influence on the fracture toughness properties (Avila 2015)
16. The role of crystallographic textures cannot be ignored because crystallographic textures significantly influence fracture behavior and impact toughness of pipeline steels. THe {332} <113> texture causes desirable values for strength and toughness, while the {001} <110> texture damages impact toughness and should be avoided (Xiao-Long 2015)
17. a decrease in toughness is associated with an increase in lath length (or effective grain size). The best method to capture changes in teh effective grain size is by measurin aspect ratio (length to width) of each lath structure (Allred 2013)
Decrease in FT
1. SZ fracture toughness lower than BM in X80 (Tribe)
2. High levels of HI (Tribe)
3. Stir zone fracture toughness reduction with an increase in HI (Santos 2010)
4. Bonnevie et al claim that massive MA constituent morphology is detrimental for the material toughness (Santos 2010)
5. The SZ microstructre of the 0.20 mm/revolution joint presents the MA constituent associated to the upper bainite-like constituent, which has been designated as the degenerated upper bainite. Elongated MA constituent and the large amount of carbide (cementite) precipitation in this region, which explains the low fracture toughness of this microstructure (Santos 2010)
6. There is a trend toward lower fracture toughness with increasing heat input, which depends on the spinning and welding speeds, and downward force. (Avila 2015)
7. The MA (martensite-austenite) constituent is primary responsible for the low toughness of simulated CGHAZ with high values of cooling time because the large MA constituent reduces the crack initiation energy significantly. (Lan 2010)
8. The decrease in carbon content to prevent the formation of martensite-austenite (MA) constituent which can reduce the HAZ toughness seriously (Lan 2010)
9. Li et al have shown that the ICHAZ toughness does not deteriorate drastically for the API X70 pipeline steels because the size of M-A constituent is small and the content of M-A constituent is relatively little for the effect of accelerating cooling (Lan 2012)
HAZ
1. Expands with increasing HI (Aydin 2013)
2. It is likely that temperatures in the HAZ did not exceed the A3 temperature, but the temperature was sufficient to cause significant coarsening and spheroidization of the carbides to occur (Aydin 2013)
3. HAZ has the lowest hardness values due to the predominantly overtempered polygonal ferrite microstructure at elevated temperatures during FSW process (Aydin 2013)
4. Divided into high HAZ and low HAZ, high HAZ was above A1 (Avila 2015)
5. The maximum amount of M-A constituent occurs in the coarse grained heat affected zone (Lan 2012)
Cooling Rate
1. Formation of bainite at cooling rates faster than 20 C/s creates hard zone (Rose)
2. Strong linear relationship between rooling rate and lath width, with lath width decreasing with increasing cooling rate (Rose)
3. Near perfect relationship between lath width and hardness (Rose)
4. Control over both heat input and backing plate enables the same desired cooling rate to be obtained over a range of parameters (Rose)
5. Backing plate
  1. Even lower cooling rates should be possible using backing plates of lower thermal conductivity or increasing the heat input. This would expand the process window even further and achieve even higher travel speeds (Rose)
6. Higher HI results in slower cooling rate (Aydin 2013)
7. Cota et al have reported that when the cooling rate increased, the reduction in the transformation start temperature led to finder bainitic structure and an increase dislocation density, producing an increase in the ultimate tensile and yield strengths. (Aydin 2013)
8. According to Ikawa et al, the MA constituent morphology varies according to the cooling rate (Santos 2010)
9. An increase in cooling rate causes a decrease in the bainite start transformation temperature, Bs (Cota 2000)
10. It can be seen that the microstructure associated with a higher finish-cooling temperature (650C) ponsists of polygonal ferrite, granular bainite, and small particles of constituent MA, martensite and retained austenite, in the bainitic matrix (Cota 2000)
11. The microstructure associated with a lower finish cooling temperature (500C) consists of granular bainite and ferrite bainitic. In this microstructure fine-grained polygonal ferrite can be seen, with small-sized and rondomly distributed MA constituent (Cota 2000)
12. The width of bainitic ferrite laths decreases with increase in the cooling rate (Cota 2000)
13. Increase in the cooling rate or decrease in the finish-cooling temperature implies a decreasing in the volume fraction and average size of the MA islands (Cota 2000)
14. Results from the literature show that the width of the bainitic ferrite laths decreases and the dislocation density increases with decrease in the Bs temperature. Therefore, when the cooling rate increases, the reduction in the Bs temperature leads to an increase in the tensile and yield stresses (Cota 2000)
  1. Subtopic 1
15. The various kinds of bainites appear in the steel when the cooling rates exceed 5C/s (Xiao-Long 2015)
16. Cooling rate equations - UTS, yield, elongation (Cota 2000)
  1. Subtopic 1
17. The best combination of ductility and tensile strength is obtained with Cr=13.3 C/s and TFC=500C. Results from teh literature also indiciate the temperature of 500 C as the best finish-cooling temperature for a great variety of steel grades. Under these cooling conditions, the microstructure is essentially bainitic with 7 % of polygonal ferrite and MA constituent (Cota 2000)
18. For Tfc = 400C the microstructure is essentially bainitic with fine laths of bainitic ferrite with interlath MA constituent (Cota 2000)
19. HAZ cooling rates (10s, 30s,50s,90s,150s) (Lan 2010)
  1. Subtopic 1
20. Thermal effects had a much larger influence on the microstructure (Rose 2013)
21. Allred varied peak temparture....and found that the microstructure was significantly affected by the peak temperature and cooling rate (Rose 2013)
22. Very little has been done to exploit the weld anvil used in FSW to affect the heat flux exitin the workpiece (Rose 2013)
23. Backin plate thermal conductivity was found to affect the peak tool temperature. They also observed that using backin plates of lower conductivity improved the weld homogeneity (Rose 2013)
24. In this study, travel speed is confounded with heat input, with higher travel speeds occurring at lower heat input levels. Thus, the increase in cooling rate with decreasgin heat input is due to the increasing travel speed at each heat input step. This correleates with the known increase in cooling rate with increasing travel speed (Rose 2013)
25. At 1000K, the estimated heat flux into the work-piece is calculated to be 43% of total heat generated in 1018 steel using tungsten tool (Nandan 2008)
26. Metal temp props (Nandan 2008)
  1. Subtopic 1
27. Temp distribution over weld (Nandan 2008)
  1. Subtopic 1
28. In high carbon steels, FSW can result in martensite formation. To keep the volume fraction of martensite low, either the peak tempearture should be lower than A1 critical temperature, or the cooling rate should be slower than the critical cooling rate. The peak temperature decreases with increase in welding speed or decrease in rotational speed. The cooling rate decreases with decreasee in welding speed and with decrease in peak temperature. Low welding speed combined with low rotational speed produce slow coolin rate and prevent martensite formation (Nanden 2008)
29. An increase in cooling rate results in the reduction of grain size and an increase in the volume of acicular and lath ferrite, potentially resulting in an increase in strength and a decrease in toughness (Allred 2013)
30. grain misorientation (aspect ratio) as a function of cooling rate (Allred 2014)
  1. Subtopic 1
31. Peak temperature and cooling rate were found to have the largest influence of all variables (Allred 2013)
  1. Subtopic 1
  2. Subtopic 2
32. Austenitic temperature as a function of chemical composition (Allred 2013)
  1. Subtopic 1
33. Jominy tests with cooling rates greater than 136°C/s exhibited lath ferrite that closely resemble lath structures found in the weld HZ. At higher cooling rates, all Jominy peak temperatures resulted in primarily lath ferrite (Figure 4-5d-f) that have aspect ratios (Table 4-4) similar to weld HZ aspect ratios (Allred 2013)
Future work
1. Welding on backing plates of even lower thermal conductivity should be performed and try to increase travel speeds beyond what is ahieveable in X65 (Rose)
2. Further exploration is needed to understand the effect of different process parameters on cooling rate (Rose)
3. Through improved temperature measurement in PCBN tools, temperature control can be accurately used in steels. Allow investigation into how temperature affects weld props (Rose)
4. According to Bhadeshia and DebRoy [18], just few works describe the fracture toughness of friction stir welds of steels, and these studies used only elementary mechanical properties. (Santos 2010)
5. Many studies did not present the complete weld and test matrix, and therefore it is difficult for other researchers to follow trends or even draw conclusions from those reports (Avila 2015)
6. Avila may have missed the effect of Mn on X80 and MA morphology
7. The loss in toughness of the SZ at -40C needs to be better studied in order to confirm the source of the reduction of toughness (Avila 2015)
8. To date, no studies have controlled the cooling rate and peak temperature during FSW (Rose 2013)
9. Further exploration is needed to understand the effect of different process parameters have on the cooling rate. Care should be taken to prevent the confoundi nof the heat input, power, and travel speed in further studies (Rose 2013)
10. Currently, there has no been a comprehensive study investigating the relationship between weld power, travel sped, weld environment, peak temperatures, and cooling rates (Allred 2013)
11. Given current understandin of FSW, a decrease in IPM at constant power and/or RPM would lead to reduction (or possible elimination) of the HZ in FSW X65 (Allred 2013)
Testing
1. Fracture modes
  1. Subtopic 1