Material Science, The Iron Carbon Phase Diagram, Part 2 — Transcript

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1. 0:02[Music]
2. 0:10[Music]
3. 0:12welcome to the world of material science
4. 0:15my name is Professor Bonilla this video
5. 0:18concentrates once again on the Mona Lisa
6. 0:21of material science the iron carbon
7. 0:24phase diagram let's take a look at the
8. 0:26individual sections of the iron carbon
9. 0:28phase diagram for the metastable system
10. 0:31step-by-step so that you can understand
11. 0:35the processes taking place and the
12. 0:37resulting micro structures let us begin
13. 0:40by looking at the solidification range
14. 0:44leaving aside the per tactic for the
15. 0:47time being at 4.3% there is an eye
16. 0:51tactic point the carbon dissolves in the
17. 0:54material reduces the melting point of
18. 0:57pure iron from 1536 degrees Celsius to
19. 1:021147 degree Celsius we have already
20. 1:06discussed that our tactic or neo tactic
21. 1:09alloys are particularly well suited for
22. 1:12casting since they are characterized by
23. 1:15a low melting temperature and ductility
24. 1:18and do not easily
25. 1:21Saeko grade or shrink consequently the
26. 1:24current content of most cast iron
27. 1:27materials is at or slightly below 4.3%
28. 1:33homogeneous gamma solids solutions
29. 1:36develop up to a carbon content of 2%
30. 1:39this corresponds to the area of four
31. 1:43jables steels as soon as a macro
32. 1:46structure contains label right which is
33. 1:49hard and brittle the material can no
34. 1:52longer be hot or cold worked
35. 1:54it can only be cast as previously
36. 1:58explained a carbon content of 6.67
37. 2:02percent corresponds to cementite content
38. 2:05of 100% all alloys in the carbon content
39. 2:09range of 0 to 2% behave like the basic
40. 2:14type complete solubility in liquid and
41. 2:17solid state thus below the liquidus
42. 2:20formation of solid solution occurs the
43. 2:24gamma solid solutions
44. 2:26are able to grow in the melt without any
45. 2:29constraint and form elongated structures
46. 2:32with branches to the sides which are
47. 2:35referred to as dendrites these crystals
48. 2:40become richer in carbon content due to
49. 2:42increasing formation in the example
50. 2:46shown here the carbon content has
51. 2:48already increased from c1 to c2 on
52. 2:53complete solidification the macro
53. 2:56structure consists of homogeneous gamma
54. 2:59solid solutions that is to say hours
55. 3:02tonight these crystals are interstitial
56. 3:06solid solutions with carbon dissolved in
57. 3:09the in the interstitial positions of the
58. 3:12face centered cubic iron lattice when a
59. 3:15hypoeutectic alloy solidifies the
60. 3:18concentration of the solid solutions
61. 3:20increases to a maximum of two percent as
62. 3:24a temperature decreases the meld
63. 3:28approaches Point C and thus toward an
64. 3:32atty tactic composition of 4.3% cabin
65. 3:36the macro structure then consists of
66. 3:39gamma solid solutions and atactic as we
67. 3:43have already seen or tactic in the
68. 3:45binary iron carbon system consists of a
69. 3:48fine-grained mic mixture of fine gamma
70. 3:51solid solutions and cementite which we
71. 3:55refer to as little right accordingly
72. 3:58hypoeutectic alloys behave like the
73. 4:01basic type decreasing solubility in
74. 4:04solid state the resulting crystal
75. 4:06mixture precipitates the component in
76. 4:11excess during the melt when a hyper or
77. 4:13tactical oil solidifies fe3c crystals
78. 4:17precipitate to form primary cementite as
79. 4:21the temperature drops the carbon content
80. 4:24of the melt starting from high
81. 4:26concentration levels decreases and
82. 4:29approaches the atactic composition upon
83. 4:33reaching the solidus their tactic line
84. 4:35or technical a double right is formed
85. 4:38the crisps
86. 4:39mixture then consists of primary
87. 4:41cementite and label right next let's
88. 4:45look at the phase transformation in the
89. 4:47solid state which is important for
90. 4:49steels an important distinction has to
91. 4:52be made between hypo pearlitic and hyper
92. 4:56politics steel steels with a carbon
93. 5:00content below 0.8% are referred to as
94. 5:03hypo politic or hypo eutectoid steels
95. 5:08above the line GS there are unsaturated
96. 5:12homogenous gamma solid solutions with a
97. 5:15carbon content of 0.15 percent in the
98. 5:20example shown here the transition from
99. 5:23the Gummer to the alpha phase starts
100. 5:26after reaching line GS which shifts to
101. 5:30lower temperatures as a carbon content
102. 5:32increases in the second phase body
103. 5:36centered cubic ferrite forms in
104. 5:38austenite as temperature decreases the
105. 5:42percentage of ferrite increases and the
106. 5:45carbon content of austenite increases
107. 5:48toward point s whenever a hypo politic
108. 5:53steel reaches the temperature of 723
109. 5:57degrees Celsius at slow cooling rate
110. 6:00that is line PSK it consists of pro
111. 6:05eutectoid ferrite that has been
112. 6:08separated out and gamma solid solutions
113. 6:12which have not yet been transformed with
114. 6:150.8 percent carbon as it passes through
115. 6:19the line PSK the face centered cubic
116. 6:22austenite solid solution changes into a
117. 6:26body centered cubic alpha ferrite
118. 6:29inserted carbon atoms are forced out of
119. 6:33the developing alpha lettuce and into
120. 6:37adjacent areas where together with iron
121. 6:40atoms they form the inter metallic fe 3
122. 6:43c phase known as cementite the
123. 6:48microstructure of ferrite with fine
124. 6:50layers of cementite is called pearlite
125. 6:53at room temperature farad exists
126. 6:56alongside perlite Steel's with a carbon
127. 6:59content above 0.8% are called hyper
128. 7:03politic hypo eutectoid steels carbon
129. 7:07solubility in these steels decreases
130. 7:10with decreasing temperature as indicated
131. 7:13by line e s which is known as a
132. 7:16solubility or saturation line as a
133. 7:19result the carbon atoms must diffuse
134. 7:23from the gamma solid solution they move
135. 7:26to the grain boundaries where they form
136. 7:29cementite crystals in the form of
137. 7:31secondary cementite at the line PSK the
138. 7:36steel first consists of a gamma solid
139. 7:39solution with a carbon content of 0.8%
140. 7:42and a network of secondary cementite
141. 7:45then the austenite transforms into
142. 7:48polite as is also the case in hypo
143. 7:52politic steels naturally engineers are
144. 7:56not interested in the structure because
145. 7:59it is nice to look at instead we want to
146. 8:02draw conclusions about the mechanical
147. 8:04properties that result from the
148. 8:07microstructure with knowledge about the
149. 8:10various hypo and hyper political matter
150. 8:13of structures we are now able to
151. 8:15understand the development of mechanical
152. 8:18properties of steel as a function of the
153. 8:21carbon content the increase of the
154. 8:24hardest component that is cementite is a
155. 8:28linear function of the carbon content
156. 8:30this results in an almost linear
157. 8:33increase in hardness HB the percentage
158. 8:38of cementite is however not the only
159. 8:41decisive factor we also need to look at
160. 8:45its position within the microstructure
161. 8:47thus we can observe a significant
162. 8:51increase in tensile strengths RM as a
163. 8:54percentage of cementite cementite
164. 8:56increases as long as the cementite is
165. 9:00embedded in the tough ferrite in the
166. 9:03form of pearlite if grain boundary
167. 9:07cement
168. 9:07occurs the strengths does not only stop
169. 9:10increasing further but actually
170. 9:12decreases slightly as a consequence of
171. 9:16the brittle cementite phase the
172. 9:19elongation at break a is strongly
173. 9:22reduced when the carbon content
174. 9:23increases therefore if I need a steel
175. 9:27that is soft and as tough as possible
176. 9:31the carbon content should be below 0.2%
177. 9:36hypu politic steels with a carbon
178. 9:39content between 0.2 and 0.8%
179. 9:42offer a good combination of toughness
180. 9:44and strength if my component needs to be
181. 9:49as hot as possible but does not require
182. 9:52considerable toughness
183. 9:53I can choose a carbon content
184. 9:56substantially above 0.8% the carbon
185. 10:00content does not only influence the
186. 10:03mechanical properties of steel
187. 10:04it also affects the technological
188. 10:07properties of the resulting material an
189. 10:10increase in carbon content lowers of the
190. 10:13melting temperature which generally
191. 10:16improves the cast ability of the
192. 10:18material cast steel which is iron with a
193. 10:22carbon content below 2% cannot be used
194. 10:26in sin walled casting because of the
195. 10:29relatively high casting temperatures and
196. 10:31the precipitation of gamma solid
197. 10:34solutions in the melt near hypoeutectic
198. 10:38cast iron alloys exhibits excellent cast
199. 10:42ability the high melting temperature of
200. 10:46steels with low carbon content which is
201. 10:49a disadvantage for casting proves is
202. 10:53advantaged for hot workability at
203. 10:56correspondingly high temperatures this
204. 10:59facilitates greater metal forming with
205. 11:02smaller force compared to steel with a
206. 11:05higher carbon content in general soft
207. 11:09ferrite is very ductile however the
208. 11:13percentage of cementite which increases
209. 11:16with an increase in carbon content
210. 11:18reduces elongation at break and
211. 11:20necking consequently is a force and work
212. 11:24required for cold working increases with
213. 11:27a carbon content above 0.8% carbon
214. 11:31content considerable cold working is no
215. 11:34longer possible because of the brittle
216. 11:37interface cementite as a percentage of
217. 11:41cementite increases so does the cutting
218. 11:44force required and the abrasive wheel of
219. 11:48the cutting edge during material
220. 11:50machining in contrast machinability
221. 11:54considerably improves in the stable
222. 11:57system in which carbon exists it's as
223. 12:00graphite the variability of steel is
224. 12:04determined by its ability to reduce
225. 12:06stress produced during welding by local
226. 12:10heating and rapid cooling with the help
227. 12:13of micro plastic deformation this is why
228. 12:17steel with low elongation at break are
229. 12:20at least prone to cracks in general it
230. 12:24can be assumed that there is good
231. 12:26variability up to a carbon content of
232. 12:290.25 percent this does not imply that
233. 12:34Steel's where the carbon content above
234. 12:350.25 percent cannot be welded instead
235. 12:39the vulnerability of these materials
236. 12:41requires a specific set of conditions or
237. 12:44methods the hard and brittle micro
238. 12:48structures that can form at higher
239. 12:50carbon content especially martensite
240. 12:54make it possible to significantly
241. 12:56increase hardness and strength by heat
242. 13:00treating the Steel's in this connection
243. 13:02Steel's with a carbon content between
244. 13:050.2 and 0.6% are of particular interest
245. 13:09we will learn more about these macro
246. 13:12structures in the next chapter I hope
247. 13:15all of you will have an opportunity at
248. 13:18some point to admire the sheer beauty of
249. 13:21these micro structures under an optical
250. 13:24microscope thanks for your attention I
251. 13:27hope you'll be back to watch our next
252. 13:29video tutorial