Microstructure Map of 17-4 Precipitation Hardening Stainless Steel
Creators: McDonald, Anne E
Publisher: name: University of Alberta Library
Year: 2024

<p>17-4PH is a precipitation hardenable stainless steel alloy with good corrosion resistance and excellent mechanical properties. Due to this combination of properties, 17-4PH is used in a range of industries including marine, nuclear, aerospace, and paper. These wide industrial applications make 17-4PH an attractive candidate for additive manufacturing (AM) to enable on-the-spot replacement part printing at distributed work locations. To be used as an industrial AM alloy, the solidification behaviour of 17-4 PH must be well-understood.  </p>

<p>To study the solidification behaviour of 17-4 PH, samples were created using differential scanning calorimetry (DSC), wire and arc additive manufacturing (WAAM), and impulse atomization (IA).  In DSC samples that featured cooling rates from 2 - 20 °C/min, a martensitic matrix with interdendritic δ-ferrite was observed.  To create uniform, single-pass beads using a WAAM system, a stable welding region was established for 17-4PH stainless steel over a torch travel speed of 4.1 - 2.5 mm/s and a wire feed speed of 38.5 - 76.2 mm/s.  These parameters gave current and voltage parameters of 80 - 130 A and 12 - 14 V, respectively, for an overall heat input range of 200 - 650 J/mm.  The cooling rates, calculated from two-colour pyrometer data, ranged from 140 - 490 °C/s. The microstructure of the WAAM samples featured a largely columnar microstructure with a small equiaxed region at the surface of the bead.  The phases of the WAAM samples, similar to those of the DSC, consisted of martensite and δ-ferrite.  The fraction of δ-ferrite over the range of heat inputs measured was fairly uniform, with an average δ-ferrite fraction of 0.21 ± 0.09 in the WAAM samples. The IA powders had a liquid cooling rate range of 600 - 12000 °C/s.  In the IA samples, two major morphologies were observed, martensitic lathes and dendrites.  Based on EBSD data, the dendrites were identified as δ-ferrite.  At the lowest cooling rates, the powder particles were completely martensitic.  As the cooling rate was increased, the alloy did not spend enough time in the austenite forming region of solidification and reached room temperature as δ-ferrite.  Starting at cooling rates of 1900 °C/s, more than 50 % of the powder particles were fully δ-ferrite.  At the highest cooling rates, more than 90 % of the powder experienced austenite by-passing and was fully δ-ferrite.</p>

<p>To further study the solidification behaviour of the WAAM beads over the cooling rates studied, the velocity of the solidification front in the beads was calculated from the angle of solidification of the columnar microstructure.  As the heat input of the WAAM process was increased, a slight decrease in the angle of growth was observed.  As WAAM is a consumable electrode process, the wire feed speed increases as the heat input increases.  Due to this relationship, as the heat input was increased, a strong linear increase in bead area was observed.  The velocity of solidification was normalized with respect to the cross-sectional bead area and the resulting normalized velocity of solidification decreased as the heat input was increased, reflecting a slower solidification front movement as the solidification cooling rate decreased.</p>

<p>To assess the effect of these microstructural changes on the properties of the alloy, microhardness measurements were taken for all the samples.  From the DSC to the WAAM samples, the microhardness increased from 290 HV 0.5 to a maximum of 360 HV 0.5.  These samples are composed of the same microstructural phases so this increase in hardness is likely due to grain refinement from the increasing cooling rate.  The microhardness of the IA powders was around 290 HV 0.5 at all cooling rates measured.  This significant drop in hardness at higher cooling rates is due to retained δ-ferrite in samples that appear fully martensitic.</p>