Hardenability and Jominy Test

Transcription

1 German Jordanian University (GJU) Material Science Laboratory IE 2210 Summer Semester i i Experiment 9 Hardenability and Jominy Test Mahmood Hisham Shubbak Title Mark Objective /10 Introduction /15 Method /15 Result /30 Discussion and Conclusion /20 Report Neatness /10 Total Mark /100 1

2 Objective: 1. Distinguish between hardness and hardenability. 2. Understand the concept of mass effect and ruling section. 3. Perform Jominy test as a method to indicate hardenability. Introduction: One of the most important properties of materials is Hardness; which describes the resistance of a material to a permanent indentation. In steels, the value of hardness depends on many factors such as the carbon content, alloying elements and the cooling rate during hardening process. The Maximum hardness in steels is obtained by producing a fully martensitic structure. This can be done by austenitizing the steel and then quenching it. During the austenitizing treatment all of the carbides dissolve and the ferrite transforms into austenite. Quenching this structure causes the austenite to transform into Martensite. This transformation is so fast that there is no time to the carbon to diffuse out of the Martensite grains or to form carbide phases. This Martensite is very hard and also very brittle. Another important property of materials is Hardenability which describes the ability of material Martensite to be hardened in large depth, or the ability of a steel to partially or completely transform from austenite to some fraction of martensite at a given depth below the surface, when cooled under a given condition (the maximum depth of Martensite). When a thick steel component is quenched from its hardening temperature, it will take longer time to the inner core of the component to cool than for the surface layers that are in contact with quenching medium. This leads to a variation in hardness across the section of the steel component, and this mass variation in hardness is referred to as mass effect. This mass effect in plain carbon steel may be remedied to some extent by adding alloying elements to the steel, such as nickel, chromium, molybdenum, etc. these alloying elements make it possible to get a martensitic structure throughout the section of the steel component, even by oilquenching. This is one of the most important functions of alloying, but the lack of uniformity of structure and hardness in steels, can seriously affect the other mechanical properties. For this reason it becomes necessary to specify the maximum diameter of the bar of "ruling section". 2

3 The hardenability of materials can be studied by a particular test called Jominy Test; in which a heated specimen of steel (austenite) is dropped into position in a frame, and quenched by spraying a pre set standard jet of water against its lower end. The standard specimen cools very rapidly at the quenched end and progressively less rapidly towards the opposite end. When cold, a flat is groundedd along the side of the specimen and its hardness is measured every 3mm form the quenched end. Method: 1. Specimen Heating: a) Choose two types of steel bars with proper dimensions to be machined to the standard dimensions of the Jominy test specimen. One of these steel bars is made from plain carbon steel and the other is from alloy steel (here we used d2 Steel). b) Heat each of these two bars up to its austenite temperature. 2. Jominy Test Machine: c) Drop each bar into its proper position of the frame of the Jominy test device. d) End quenches each bar with tap water for about 15mins. 3. Hardness Measuring: e) Grind a flat along the side of each specimen. f) Measure the hardness every 3 mm form the quenched side of the bar using Vickers 1 hardness testing machine (the oldest one in the world). g) Plot the hardness measured (HV) of each bar with the relation to the distance from quenched end (mm) on the same graph paper. h) Compare the hardenability of the two obtained curves. Results: Applying the experiment method we got the following results: 1 In the standard Jominy test Rockwell C Hardness must be used, but here in this experiment we have used Vickers instead. 3

4 Distance d1 d2 avg HV Specimen Type: Plain Carbon Steel Load (kg) Hardenability for Plain Carbon Steel [1045] HV أسي. HV)) Distance from quenched end (mm) ) rs Hardness (HV) Vicke

5 Distance HV Specimen Type: d2 Steel Hardenability for d2 Steel HV أسي. HV)) Distance from quenched end (mm) Vickers Hardness s (HV)

6 Distance Plain Steel d2 Steel Plain Steel d2 Steel أسي. Steel)) Plain أسي. Steel)) d Vic ckers Hardness (H HV) Hardenability for 1045 & d2 Steel Distance from quenched end (mm)

7 Discussion and Conclusion: In this experiment we learned about the Hardenability of Steels. Hardenability is an important property of material (especially steels) that describes the capacity of the steel to harden in depth under a given set of conditions. Jominy test is an effective method to measure Hardenability of steels; by quenching one end of a specimen to generate Martensite along its depth, then measuring the hardness at different distances from this end, and determine the ability of this steel specimen to generate Martensite with depth. As we have studied (and also noticed in the lab) adding some alloys to steel make it more hardenable. The following graph explains this: When we applied the Jominy test on the two specimens in this experiment we found the following: 1. The 1 st specimen's (Plain Steel 1045) curve was approximately similar to our expectations except some out of range points as you can see in the results part of this report. 2. The 2 nd specimen's (D2 Steel) curve was almost strange; since the first 12 points are less hard than the next part of the curve, and this doesn t make sense (because these 1 st points have more cooling rate i.e. more opportunity to form Martensite). 3. But in general we can notice from the third graph (the comparison one) the effect of alloying in increasing hardenability. This unexpected curve 2 (of D2 steel) can be due to some factors such as: a. Some inhomogeneousity in the specimen. 2 Notice: this part of experiment for D2 steel has done by the other group of students, and I actually don t know the way or conditions in which they made it. So all the reasons I mentioned are just expectations. 7

8 b. Mistakes in applying Jominy test (especially in the rate of cooling along distance). c. Mistakes in measuring hardness. I think that the third reason is the most expected because they have changed the hardness testing machine (from the manual one to the automatic one) between the 12 th and 13 th points, and their mistakes could be either in measuring d1 & d2 of the impression, or in calculating the Vickers hardness. Alloying usually increases the hardenability of steels because it increases the time needed to form martensite (decreases the critical cooling rate). From this experiment we can conclude that the plain carbon steel 1045 has less hardenability than D2 steel. This property (hardenability) can help in choosing suitable metals for more effective heat treatment and performance in industrial applications. 8