実験報告書様式（一般利用課題・成果公開利用）

MLF Experimental Report
課題番号 Project No.

提出日 Date of Report
March 8, 2011
装置責任者 Name of responsible person

2010B0019

実験課題名 Title of experiment

Kazuya Aizawa

In situ neutron diffraction and dilatometric measurements for

装置名 Name of Instrument/（BL No.）

tempering behavior of martensite steels

Engineering

実験責任者名 Name of principal investigator: Yo Tomota
所属 Affiliation: Ibaraki University

materials

diffractometer(BL-19)
実施日 Date of ExperimentDec.10 – Dec.14, 2010

試料、実験方法、利用の結果得られた主なデータ、考察、結論等を、記述して下さい。（適宜、図表添付のこと）
Please report your samples, experimental method and results, discussion and conclusions. Please add figures andtables for better explanation.
1. 試料 Name of sample(s) and chemical formula, or compositions including physical form.
The chemical compositions of high nitrogen martensitic steel are 0.1C-0.6N-16Cr-1.0Mo-0.6V (mass %). The
steel was melted in a nitrogen atmosphere and an ingot was forged to a plate. Small specimens cut from theplate were homogenized and solution treated at 1423K for 1.8ks followed by quenching into water. The
microstructure of the as-quenched specimen consists of martensite and the retained austenite. Another steel
with chemical compositions of 0.79C-1.51Si-1.98Mn-0.98Cr-0.24Mo-1.06Al-1.58Co (mass%) was also used forreference. For this steel, the microstructure was adjusted to be bainitic ferrite with the retained austenite.
The dissolution of austenite with tempering was common objective to be made clear.

2. 実験方法及び結果 （実験がうまくいかなかった場合、その理由を記述してください。）
Experimental method and results. If you failed to conduct experiment as planned, please describe reasons.
Thesizeofaspecimen

usedforsimultaneousmeasurement of dilatometry and neutron diffraction was 5by 5 by 30 mm (rod). A dilatometer was set at theStart specimen : 1150℃ 30min holding
and water quenching

Engineering materials diffractometer (TAKUMI) in such a
70min 700℃

way that the longitudinal direction of the rod specimen was
70min

45 degrees with respect to the incident neutron beam.
Hence, the data along the longitudinal and transverse
directions were obtained at the north and south detectorbank. In order to examine microstructure change withtempering, a specimen was heated step by step like in Fig. 1.

600℃

70min
500℃
70min
400℃
70min
300℃
70min
250℃
70min
200℃
70min
RT

Furnacecooling

A little different heating schedule was also examined. Incase of bainitic steel, the specimen heated up to 900℃

Fig. 1 Heating schedule for tempering

were cooled down to 300℃ and kept there to observe

study for the high nitrogen bearing steel.

bainitic transformation..
12. 実験方法及び結果（つづき） Experimental method and results (continued)
Examples of diffraction profiles obtained areγ(311) α(211) γ(220) α(200)presented in Fig. 2. As seen, the startingmicrostructure is composed of martensite andγ(200) α(110) γ(111)Beforeright hand side because of thermal expansion.
The FWHM was decreased at 600 ℃ andbecame much smaller after heating indicating

Intensity(a.u.)austenite. At 600℃, all the peaks shift toward
600℃

After

the tempering of martensite. The precipitation

Cr2Nof Cr nitride was found at 600℃. To be noted
here is the decomposition of the retained

101520TOF(ms)austenite as is summarized in Fig. 3. In case of

32530x10Fe-C martensite, the retained austenite isFig. 2 Example of neutron diffraction profiles

decomposed to ferrite and cementite aroundobtained

280 ℃ . However, the present high nitrogen
400℃ and starts to decompose around 500℃..
The decrease in FWHM for martensite peaks
suggestthattransformationinduceddislocations in the quenched martensite areIntensity(a.u.)bearing austenite is stable even heating at

more difficult to be annihilated than in theconventional carbon martensite. This high504540
3530
25
20
15
1050α(200)γ(200)

RT200

nitrogen steel is characterized by showing thesecondary hardening by tempering as isobserved in Fig. 4, so that high strength can beachieved at higher temperature like 500℃. The
secondaryhardeningiscausedbythe250

300

400

500600700 RT_afterTemperature (℃）
Fig. 3 Diffraction intensities as a function oftemperature for 200 martensite (or ferrite) and
200 austenite.

precipitation of nano-sized Cr nitride, that wasscattering study. But the present data claim thehighdensityofdislocations

athighertemperature contributes also to suppressforbutmore

400
300

00 RT 100

example,expansion corresponding to the austenite
decomposition,

500

100

The change in specimen length informs us
evolution,

600200softening with tempering.
microstructure

700Hardness (HV)

confirmed by means of small angle neutroncomprehensive

information can be obtained from neutron

200

300

400

500

600

700

Temperature (℃)

Fig. 4 Hardness change with tempering (1h at
each temperature).

diffraction from a bulk specimen.
2