Context :

Aims : The iron abundance gradient in the Galactic stellar disk provides fundamental constraints on the chemical evolution of this important Galaxy component .
However the spread around the mean slope is , at fixed Galactocentric distance , larger than estimated uncertainties .

Methods : To provide quantitative constraints on these trends we adopted iron abundances for 265 classical Cepheids ( more than 50 % of the currently known sample ) based either on high-resolution spectra or on photometric metallicity indices .
Homogeneous distances were estimated using near-infrared Period-Luminosity relations .
The sample covers the four disk quadrants and their Galactocentric distances range from \sim 5 to \sim 17 kpc .
We provided a new theoretical calibration of metallicity-index-color ( MIC ) relation based on Walraven and NIR photometric passbands .
We estimated the photometric metallicity of 124 Cepheids .
Among them 66 Cepheids also have spectroscopic iron abundances and we found that the mean difference is -0.03 \pm 0.15 dex .
We also provide new iron abundances , based on high-resolution spectra , for four metal-rich Cepheids located in the inner disk .
The remaining iron abundances are based on high-resolution spectra collected by our group ( 73 ) or available in the literature ( 130 ) .

Results : A linear regression over the entire sample provides an iron gradient of -0.051 \pm 0.004 dex kpc ^ { -1 } .
The above slope agrees quite well , within the errors , with previous estimates based either on Cepheids or on open clusters covering similar Galactocentric distances .
However , Cepheids located in the inner disk appear systematically more metal-rich than the mean metallicity gradient .
Once we split the sample in inner ( R _ { G } < 8 kpc ) and outer disk Cepheids we found that the slope ( -0.130 \pm 0.015 dex kpc ^ { -1 } ) in the former region is \approx 3 times steeper than the slope in the latter one ( -0.042 \pm 0.004 dex kpc ^ { -1 } ) .
We found that in the outer disk the radial distribution of metal-poor ( MP , [ Fe / H ] < -0.02 dex ) and metal-rich ( MR ) Cepheids across the four disk quadrants does not show a clear trend when moving from the innermost to the external disk regions .
We also found that the relative fractions of MP and MR Cepheids in the 1st and in the 3rd quadrant differ at 8 \sigma ( MP ) and 15 \sigma ( MR ) level .
Finally , we found that iron abundances in two local overdensities of the 2nd and of the 4th quadrant cover individually a range in iron abundance of \approx 0.5 dex .

Conclusions : Current findings indicate that the recent chemical enrichment across the Galactic disk shows a clumpy distribution .