9.5 CASE DEPTH MEASUREMENTS IN STEELS
Case depth (CD) is defined as “the perpendicular distance from the surface of the steel to the point at which the change in hardness, chemical composition or microstructure of the case and core cannot be distinguished”. This is the total case depth of steel.
The effective case depth of steel is the perpendicular distance from the surface to that point of core at which hardness values are given in table 9.2.
Table 9.2 Critical Hardness Values for Defining Case Depth
Type of steels
1. With %C 0.28-0.32 0.33-0.42 0.43-0.52 Over 0.53 2. Case hardening
3. Nitriding
4. Carbonitriding
Hardness (VHN)
Processes
Flame or induction hardened
350 400 450 550 550 Carburized, hardened and
tempered
Values to be mutually agreed Nitrided and carbonitrided upon
Values to be mutually agreed Nitrided and carbonitrided upon
Case depth is one of the deciding factors for a steel to be used in practice. Hence, it is necessary to measure the case depth of the steel so that the component can be used in service safely. There are four methods of measuring case depth:
Each Segment Focusing Mirror Consists of
Reflects Only Individually Prealigned
Focusing Mirror Pert of Beam Segments Oscillating
Mirror
Incoming Laser Beam
Laser Beam
Superimposed 45°Mirror Segments of
45°Mirror Laser Beam Painted Oscillating Mirror
Area Work-piece Treated Area Work-piece
(a) (b)
Fig. 9.4 Beam manipulation technique used for laser. (a) Segmented mirror system.
(b) Two-axis vibrator system.
(i) Hardness method (ii) Chemical method
(iii) Macrostructure method (iv) Microscopic method.
These methods measure the case depth of the steel which is surface hardened by carburizing, nitriding, carbonitriding, cyaniding or induction and flame hardening processes. 9.5.1 HARDNESS METHOD
In this method, hardness values are taken along the case and core. It is a very accurate method since sharp change in hardness across case and core region can be measured. Specimens for this method are prepared by (i) cross-section procedure, (ii) taper-grind procedure, and (iii) step-grind procedure (IS: 6396), which we now discuss. Cross-section Procedure
The specimen is cut perpendicular to the hardened surface at a predecided area. Care should be taken to ensure that no change in hardness takes place as a result of cutting. The surface area is polished up to 4/0 emery paper.
Figure 9.5 gives the method of measuring case depth for light and medium cases, while Fig. 9.6 shows the method of measuring the case depth for medium and heavier cases. The distance of the center of each impression (formed by hardness measurement) is measured with the help of a suitable optical instrument which is calibrated.
L
Case Case
CD
θ Core CD＝Lsinθ
Fig. 9.5 Methods for measuring light and medium cases. Fig. 9.6 Methods for measuring
medium and heavy cases.
Core
Taper Grind procedure
This method is suitable for light and medium cases, and is illustrated Fig. 9.7. L
Case
A shallow taper is ground through the case CD of steel, and the hard ness values are Core CD＝Lsin θ found along the surface of the prepared
taper. Care should be taken to avoid Fig. 9.7 Taper ground specimen for light and medium case. reheating, cracking and tempering of steel during cutting while preparing specimen.
Case
Step Grind Procedure
For medium and heavy cases, step Core grind procedure is suitable. In this case, the hardness values are taken in steps of Fig. 9.8 Step ground specimen for medium and heavy case. known distances below the surface. Figure 9.8 illustrates the step grind
method. Here, two predetermined depths are ground to ensure that the effective case depth is within specified limits.
For all the three procedures we have discussed, special care must be taken
during hardness measurement. It is advisable to use such hardness testers whose indentor gives shallow impression so that the hardness values obtained are representative of the surface of the case or core. Testers which produce diamond pyramid having 10 hardness numbers are recommended. Rockwell A and C scales are recommended only for such steels which have high case depth.
9.5.2 CHEMICAL METHOD
It is a usual practice to apply this method only to carburized cases. It may also be applied to other case-hardening methods which involve change in chemical composition, viz. nitriding and carbonitriding. Carbon content is determined at various depths below the surface of case-hardened specimen. Where hardening is due to formation of nitrides (e.g. nitriding), nitrogen content is estimated. This is the most accurate method of determining case depth.
9.5.3 MACROSTRUCTURE METHOD
In this technique, the test specimen is cut perpendicular to the hardened surface. The cut surface is ground and polished through 0/0 to 4/0 emery paper. After this, disc polishing is carried out, which is followed by etching with suitable etching reagent so that case and core regions can be distinguished. The etched specimen is washed in water or alcohol and dried, and is examined under a limited magnification of 320. 9.5.4 MICROSCOPIC METHOD
In this method, the specimen is cut perpendicular to the hardened surface and the cut surface is ground, polished and etched. The specimen is examined under microscope with calibrated eye piece. The total case depth is the distance from the surface to the point up to which the modified microstructure is observed.
第六章 作业一
1．什么叫表面淬火？其目的是什么？常用的表面淬火方法有哪
两种？
9.1 Compare the processes of flame hardening with induction
hardening.
2．钢材经感应淬火后的性能与整体淬火相比有何差异？
9.3 Specify the composition of steels recommended for
induction hardening.
（ 9.2 What is a laser hardening process? Discuss the basic
difference between laser heat treatment process and other conventional processes. ）
相关推荐：
loading