Question
How does the dynamic fracture toughness of four steel grades prepared through various methods compare?
1
Background T e ability of a high
strength cast steel to resist brittle fracture is a function of many diff erent metallurgi-
cal factors, including inherent matrix toughness, alloy composition, and the morphology and distribution of second phase particles such as nonmetallic in- clusions. For steels of similar strength and microstructure, fracture tough- ness mostly depends on cleanliness. In this regard, deoxidation practice and addition of titanium to remove nitrogen can aff ect inclusion type, size, morphology, and distribution and the fracture behavior. Titanium readily forms coarse carbonitrides. In
2 HY130 - A
HY130 - B HY130 - C
HY130 - D HY130 - E 4325 4130 Fe-Mn-Al-C
Procedure Five heats of HY130
(5% nickel) steel were pre- pared to study the eff ect of diff erent deoxidation prac-
tices as shown in Table 1. HY130 steels were compared to a 4130 (0% nickel), 4325 (2% nickel) and Fe-Mn-Al-C steel heat treated to approximately 36 HRC. Heats were poured into bonded
Table 1. Deoxidation Practice Alloy/Heat
Quantity/Furnace
140 lbs./induction furnace
100 lbs./ induction furnace
20 lbs./vacuum induction furnace
1000 lbs./ induction furnace
200 lbs./induction furnace
induction furnace (size not reported)
induction furnace (size not reported)
200 lbs./induction furnace
addition, titanium and aluminum can promote eutectic, type II, manganese -sulfi des which lower toughness. Ad- dition of misch metal has been shown to convert type II MnS into globular type I MnS and restore toughness. Quench-hardened and tempered
chromium and molybdenum steels are used extensively for structural applications, and the refi ned grain structure of the steels helps suppress brittle fracture. Addition of nickel to improve fracture resistance of steel has a rich metallurgical history, and nickel is one of the few alloy additions that eff ectively lowers the ductile-to-brittle transition temperature. HY130 steels are high strength chromium and mo- lybdenum steels that have additions of nickel that typically range between 4.75 and 5.25% nickel by weight. Austenitic steels in the Fe-Mn-Al- C system have gained much inter-
sand molds. T e chromium and mo- lybdenum castings were austenitized at 1,546F (841C), quenched, and tem- pered for one hour to produce similar microstructures and hardness levels. Fe-Mn-Al-C specimens were solu-
tion treated for two hours at 1,922F (1,050C), water quenched, and age hardened from 13-60 hours at 986F (530C) to HRC 32 and HRC 38.
est as a lightweight alternative to traditional high strength cast steels. An austenitic matrix and high work hardening rates contribute to high energy absorption rates under impact loading. T e mechanical properties of Fe-Mn-Al-C steels depend on the composition and degree of age hardening. At a base composition of Fe-30Mn-9Al-1Si-0.9C-0.5Mo adding aluminum lowers the density by almost 15% when compared with chromium and molybdenum steels. In their recent study, the Missouri
Science and Technology research- ers found the dynamic fracture toughness of three chromium and molybdenum steels with 0, 2 and 5% nickel with regard to deoxidation practice. Results were compared with the dynamic fracture toughness of a Fe-30Mn-9Al-1Si-0.9C-0.5Mo steel aged to a similar hardness.
Dynamic fracture toughness speci- mens were machined from the center of the castings as rectangular Charpy bars (10 x 10 x 55 mm) and fatigue pre-cracked in 3-point bending as per ASTM 1820. Finished dynamic frac- ture toughness bars were fractured at room temperature on a Tinius Olson Charpy impact machine outfi tted with an instrumented striker.
Atmosphere
Filter Argon (Ar) gas cover No fi lters used Ar gas cover
Vacuum of 1 torr SPAL (surface
protective argon liquid)
No protective atmosphere
Ar gas cover Ar gas cover Ar gas cover 46 | METAL CASTING DESIGN & PURCHASING | Jul/Aug 2012 64 ppi fi lter used No fi lters used No fi lters used
Deoxidation Practice Aluminum (Al) and Calcium (Ca) Al and Ca Al and Ca Al, Titanium (Ti) and misch metal. No fi lters used Pre-alloyed ingot was remelted; Ti was added No fi lters used No fi lters used No fi lters used Al Al, Ca and Ti No deoxidation required
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60